• Preheat the oven to 350°F.
• In a stand mixer, combine the cake mix, sour cream, oil, water, sugar, eggs and vanilla.
• Blend the ingredients together on low speed for one minute.
• Using an ice cream scoop or spoon, scoop out 24 mounds of the batter onto cookie sheets lined with parchment paper.
• Bake for 10 to 12 minutes until the tops spring back when touched. Remove and let cool completely.
• Next, prepare the buttercream by bringing a large pot with an inch or two of water to a boil over high heat.
• Combine the egg whites and sugar in a smaller pot and place it over the larger pot of boiling water. Whisk the ingredients together just until the sugar is dissolved and the temperature reaches 180ºF on an instant-read thermometer. Transfer the mixture immediately to a stand mixer fitted with the whisk attachment.
• Beat the dissolved sugar mixture on high speed until it doubles in volume and becomes thick and shiny. Continue to whisk until cool.
• Reduce speed to medium and add butter, roughly 1 Tablespoon at a time, until it is all incorporated.
• Add the vanilla and salt (if mixture looks curdled, continue to whisk until smooth).
• Increase the speed to high and whisk for 1 more minute.
• Assemble the whoopie pies by spreading a portion of the buttercream onto the flat sides of twelve of the cakes. Top with a non-filled second cake to form sandwiches. Enjoy them plain or rolled in sprinkles or nuts.

Kelly's Note:

• If the buttercream is too thin, pop it in the freezer for 10 to 15 minutes to thicken it.
• ★ Did you make this recipe? Don't forget to give it a star rating below!