Monster Cookie Bars

from 8 votes

These soft and chewy Monster Cookie Bars are a flourless, gluten-free dessert loaded with peanut butter, oats, M&Ms and chocolate chips.

Monster cookie bars studded with M&Ms and chocolate chips cut into triangles.

Monster Cookies, meet your culinary cousin, Monster Cookie Bars. These peanut butter oatmeal cookie bars are denser, chewier, and more durable than their drop-cookie counterpart, making them perfect for on-the-go snacking or packing in a lunchbox. Both desserts are flourless and both desserts contain a whole lot of peanut butter and chocolate. They’re also loaded with old-fashioned oats, which add a hearty texture and a touch of nuttiness to the bars.

What sets these cookie bars apart is how simple they are to make. You don’t have to worry about scooping or shaping the dough like you would with regular chocolate chip cookies cookies. You just press the dough into a baking pan and let them bake until they’re a beautiful golden brown color. That’s it!

  • Thick and chewy.
  • Made in 1 bowl.
  • Loaded with peanut butter, M&Ms and chocolate chips.
  • Naturally gluten-free (if you use all gluten-free certified products).

Grab These Ingredients

Glass bowls containing ingredients to make monster cookie bars, including M&Ms, chocolate chips, baking soda, white and brown sugars, vanilla extract, peanut butter, eggs and old-fashioned oats.
  • Sugars: You’ll need good ol’ granulated and light brown sugars to add sweetness and moisture to the bars.
  • Butter: Soften it to room temp for easier mixing and a smoother texture in your cookie dough. If you forget to take it out in advance, you can gently soften it in the microwave in short bursts, being careful not to melt it completely.
  • Eggs: Bind the ingredients together and add structure.
  • Peanut butter: Smooth or chunky peanut butter both work well in this recipe. However, I’d recommend steering clear of natural peanut butter, as it can lead to crumbly bars. Just like when making peanut butter chocolate chip cookies or no-bake peanut butter bars, I recommend sticking to processed peanut butter (like Skippy or Jif) for the best results.
  • Vanilla extract: For flavor. Homemade vanilla extract is a great option to enhance the rich taste of these treats even further!
  • Baking soda: Acts as a leavening agent to help the bars rise and become fluffy.
  • Salt: Enhances the overall flavor and balances out the sweetness. If you’re using salted butter, I recommend you reduce or omit the additional salt.
  • Quick oats: You can use old-fashioned oats as a substitute, but the texture may be slightly different.
  • M&Ms and chocolate chips: These classic additions add gooey pockets of melted chocolate throughout the bars. Feel free to mix and match with your favorite varieties of chocolate chips (milk chocolate, semi-sweet or dark chocolate chips would all be delicious) or candies to personalize your monster cookie bars.

See the recipe card for full information on ingredients and quantities.

All you need is one bowl to whip up these flourless monster cookie bars. I use my stand mixer but a large bowl and a hand mixer (plus some elbow grease!) works, too. 

  1. Preheat your oven to 350°F and prepare your baking pan. Line a 9×13-inch metal baking pan with parchment paper, leaving a 2-inch overhang, then grease it with cooking spray. Secure the overhanging parchment paper with all-metal binder clips to keep the paper in place during baking.
  2. Cream together the butter and sugars in a mixing bowl. Scrape down the sides of the bowl periodically to ensure everything is well mixed. Continue mixing for about 2 minutes until the mixture transforms from a darker brown color to a light brown hue and achieves a fluffy consistency.
  3. Add the eggs to the bowl, one at a time, beating well after each addition.
  4. Add the peanut butter, vanilla extract, baking soda, and salt and mix until well combined.
  5. Mix in the oats just until combined.
  1. Gently fold in the M&Ms and chocolate chips to evenly distribute them throughout the dough.
  2. Transfer the cookie dough to your prepared pan. Use a spatula or the back of a spoon to press the dough into an even layer.
  1. Bake the bars for 25 to 30 minutes until a toothpick inserted comes out clean. Keep an eye on them towards the end of the baking time to avoid overbaking. They should be golden brown and a toothpick inserted into the center should come out mostly clean with a few moist crumbs.
  2. Cool in the pan for 1 hour. This helps them set properly and makes them easier to slice. Be patient and resist the temptation to cut them too soon!
A hand holding a sharp knife cuts peanut butter oatmeal bars studded with M&Ms and chocolate chips into perfect triangles.

They’re packed with a whole lot-o-protein courtesy of peanut butter. And let’s be honest, they’re not healthy, but the lack of flour and the inclusion of oats makes me think these are suitable for breakfast or dessert. #nojudgementzone

Kelly’s Recipe Notes

  • These peanut butter oatmeal cookie bars are definitely on the thicker side. If you prefer thinner cookies (that will be crispier and less chewy), you can use a bigger pan.
  • For chewier, softer cookie bars, underbake the cookie and allow it to cool in the pan for 30 to 45 minutes before slicing and serving.
  • If you are using a glass 13×9-inch baking pan, you may need to decrease the oven temperature to 325°F based on how the heat circulates in your oven.
  • Jazz up these monster cookie bars by using festive holiday M&M’s to match the occasion. Whether it’s Christmas, Halloween, or any holiday in between, the colorful M&M’s add a fun and festive touch to your treats.

Storage Tips

Flourless monster cookie bars will last for up to 5 days when stored in an airtight container at room temperature.

They also freeze really well and can be stored in an airtight container in the freezer for up to 3 months. Just make sure to wrap them tightly in plastic wrap or aluminum foil before freezing. When you’re ready to enjoy them, simply thaw the cookie bars at room temperature for a few hours and they will be ready to serve.

Common Questions

Can I use a different nut butter?

Yes, you can definitely use a different nut butter to make monster cookie bars! You can use almond butter, cashew butter or even sunflower seed butter if you have a nut allergy. Just keep in mind that using a different nut butter could slightly alter the taste and texture of the cookie bars.

Can I add nuts or other mix-ins to these bars?

Absolutely! Feel free to customize these bars with your favorite cookie mix-ins, just like you would when making Kitchen Sink Cookies.

Peanut butter oatmeal bars with M&Ms and chocolate chips.
Craving more? Subscribe to Just a Taste to get new recipes, meal plans and a newsletter delivered straight to your inbox! And stay in touch on Facebook, TikTok, Pinterest and Instagram for all of the latest updates.
Soft and chewy flourless Monster Cookie Bars loaded with peanut butter, oats and chocolate.
Author: Kelly Senyei
4.38 from 8 votes
Monster cookie bars studded with M&Ms and chocolate chips cut into triangles.
Prep Time 15 minutes
Cook Time 35 minutes
Total Time 50 minutes
Servings 24 bars


  • 1 cup sugar
  • 1 cup brown sugar
  • 1/2 cup unsalted butter, at room temperature
  • 3 large eggs
  • 1 1/2 cups peanut butter (smooth or chunky)
  • 1 Tablespoon vanilla extract
  • 2 teaspoons baking soda
  • 1/2 teaspoon salt
  • 4 1/2 cups Quick oats
  • 1 cup M&Ms
  • 1 cup chocolate chips


  • Pre-heat the oven to 350°F and line a 13×9-inch metal baking pan with greased parchment paper with a 2-inch overhang on each short side of the pan.
  • In the bowl of a stand mixer fitted with the paddle attachment, cream together the sugar, brown sugar and butter until light and fluffy, about 2 minutes.
  • Add the eggs to the bowl, one at a time, beating between each addition then add the peanut butter, vanilla extract, baking soda and salt and mix until well combined.
  • Add the oats and mix just until combined. Stir in the M&Ms and chocolate chips.
  • Transfer the cookie dough into the prepared pan, pressing it into an even layer. Bake for 25 to 30 minutes until a toothpick inserted comes out clean. (See Kelly's Notes.) Remove the pan from the oven and allow the cookie to cool in the pan for 1 hour. Using the parchment paper overhang, lift the cookie out of the pan and onto a cutting board. Slice the cookie into 24 bars and serve.

Kelly’s Notes

  • For chewier, softer cookie bars, underbake the cookie and allow it to cool in the pan for 30 to 45 minutes before slicing and serving.
  • If you are using a glass 13×9-inch baking pan, you may need to decrease the oven temperature to 325°F based on how the heat circulates in your oven.
  • ★ Did you make this recipe? Don’t forget to give it a star rating below!


Calories: 343kcal, Carbohydrates: 41g, Protein: 7g, Fat: 17g, Saturated Fat: 6g, Cholesterol: 33mg, Sodium: 236mg, Potassium: 179mg, Fiber: 2g, Sugar: 29g, Vitamin A: 185IU, Vitamin C: 0.1mg, Calcium: 45mg, Iron: 1.3mg


Did you try this recipe?

Share it with the world! Mention @justataste or tag #justatasterecipes!

This post may contain affiliate links.

Recipe by Kelly Senyei of Just a Taste. Please do not reprint this recipe without my permission. If you'd like to feature this recipe on your site, please rewrite the method of preparation and link to this post as the original source.

Feeling social?

Share this recipe!


Related Recipes

Check out more crave-worthy favorites

Kelly Senyei holding a copy of The Secret Ingredient Cookbook

love the recipes on just a taste?

Check out my cookbook!

Order your copy of The Secret Ingredient Cookbook featuring 125 brand-new family-friendly recipes with surprisingly tasty twists!

This post may contain affiliate links.

4.38 from 8 votes (2 ratings without comment)

Join the Conversation

Rate and Comment

Your email address will not be published. Required fields are marked *

Recipe Rating


  1. We LOVE these bars!! I always put them in a large 11×17 baking sheet and only bake for 15-20 minutes and they are sooo chewy and yummy! Thank you!!

  2. 5 stars
    Always get positive feedback and a request for the recipe when i bring these to a party. My family loves eating the edges and corners that I cut off before serving.

  3. 5 stars
    I used to make these all the and take them to work as a treat for my co-workers, they loved them! Since retiring, my grandkids and dil beg me to bake them. They turn out wonderfully every time!

  4. I too had problems getting these to cook all the way. 30 mins and still very wet on the inside. Going on 50 mins now and still not done. Hoping they don’t completely burn on the edges.

    1. I’m sorry you didn’t enjoy the recipe, Kristin. Ovens and baking pans can cause discrepancies in baking times however, I may need to rework this recipe.

  5. 3 stars
    I wish that I had read the comments before making these… I baked mine according to the instructions and they are doughy in the middle. However they looked done on the outside! I’d probably recommend halving the recipe like mentioned above, or spread it out on a larger sheet. It’s a bit much for a 9 x 13 pan. :(

    1. Thank you for your comment, Cassandra. I’m sorry you didn’t enjoy the recipe. Ovens and baking pans can cause discrepancies in baking times.

    1. Hi there! Every recipe on the site includes estimated nutritional info. All you have to do is click on the purple button below the recipe that says ESTIMATED NUTRITION INFORMATION.

  6. 3 stars
    My daughter made this recipe last night. The cooking time said 30 mins at 350 degrees but less for chewy bars in a 9″x13″ pan. I was concerned on cooking time with such thick bars but she followed the instructions as stated. She baked for 26 mins. They were not done. In the end she cooked for an additional 20 mins with no luck. The clean toothpick trick doesn’t work well either when you have melted chocolate that sticks to it. They smelled & looked wonderful, sides more done than center. Our oven is fairly new. She also didn’t realize to save more M&M’s back for the top decorations. She felt it was a waste of her time & money since it is costly to make. Not a good choice to make for an inexperienced baker.

    1. Hi there! I’m sorry you didn’t enjoy the recipe. Ovens and baking pans can causes discrepancies in baking times.

  7. I make the full recipe but bake it in my Pampered Chef rectangle cookie sheet. The are the right thickness. I set the time for 20 minutes and go from there as to how long I bake it. My grandkids ask for these any time they need cookies for something.

  8. For anybody who doesn’t maybe want them quite as thick (they bake up to pretty much fill the depth of the 9X13), I tested this out the second time I baked these… Make half the recipe (the only thing that doesn’t halve easily is the eggs–I just put in 2 eggs) and still put it in the 9X13. I baked mine for 20 minutes then, and they were perfect! Still get to experience the deliciousness of this recipe but in a thickness more common to most cookie bars. We love them!
    Kelly, this really is an amazing recipe! Thanks so much for sharing it!

    1. Hi there, Nita! That should work, although it may alter the texture. I would add it at the same time you add the oats. Let me know how it turns out if you give it a shot!

    1. Hi Nyda! You can use another type of nut butter, such as almond butter, however the texture might be slightly different.

  9. Can I use quick oats instead of old fashioned cause that’s all I have and I have a bunch of it? Also if I use a jelly roll pan will they come out chewy still ?

  10. Hi Kelly, these are definitely my new go to recipe also. I don’t even need the recipe’s “all in the vault”. It’s so much nicer making bars as opposed to cookies, whole lotta wasted space between those cookies don’tcha know. I made the original a few times…AWESOME!! Then I added a tbs of cinnamon…also delish! I also subbed granola for the oats..also delish…But..the coo de however you spell it, was subbing Cinnamon Toast Crunch for the oats…seriously The BOMB!! Try it sometime and Thanks for all the great recipes. =) Hargo

    1. Love all of these twists on the recipe, Hargo! Thanks for sharing and so glad you’re enjoying the recipe :)

  11. I put my cookie bars in the oven for 40 minutes but when i stick a toothpick in it still comes out unclean. What should I do?

  12. Hi Kelly! That was really cool! Can’t wait for making this cake. Thanks for the recipe!
    I want to ask you something,what is the size of cup that you use ?
    And the oatmeal, can i use with a different brand?

  13. Kelly! I have had such good luck with your recipes, so I was so excited to try this one tonight. I can’t figure out what happened, but I know I used the exact ingredients listed (Jif PB) and used a 10×15 Pyrex pan to make them thinner, and after 25 minutes the tester looked clean, but after sitting for 20 minutes they were practically goo in the middle. I put them in the oven again for 25 for minutes and they still don’t look done. Any ideas? They smell delicious, at least. :)

See More Comments