In a medium bowl, whisk together the flour, baking soda and salt.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, brown sugar, white sugar and vanilla extract until light and fluffy, about 2 minutes, scraping down the sides as needed. Add the eggs, one at a time, beating well after each addition. Add the flour mixture, beating to combine, then stir in the M&Ms.
Cover the bowl with plastic wrap and refrigerate the dough for a minimum of 2 hours and up to 3 days. (See Kelly’s Notes.)
When ready to bake, preheat the oven to 375°F and line two baking sheets with parchment paper.
Drop 2- to 3-tablespoon mounds of dough onto the baking sheets, spacing the mounds at least 2 inches apart.
Bake the cookies for 9 to 12 minutes, rotating the baking sheets halfway through, until the cookies are golden brown and barely set in the centers. Cool the cookies on the baking sheets for 5 minutes then transfer them to a wire rack to cool completely. (See Kelly’s Note.)