In a large bowl, whisk together the flour, baking soda, and salt. Set aside.

In the bowl of a stand mixer with a paddle attachment (or a handheld mixer), beat the butter on medium-high speed until smooth (about a minute).

Add both sugars and mix on medium-high speed until smooth (about 2 minutes).

Add the eggs and vanilla, beating until combined. Scrape down the sides of the bowl as needed.

Add the flour mixture to the butter mixture and mix on low until combined.

With the mixer still on low (or by hand), add the M&M's and mix until incorporated. (If using an electric mixer, only mix for a few seconds.)

Cover the bowl and let the dough chill in the refrigerator for at least 2 hours, up to overnight.

When ready, remove the dough from the refrigerator and allow it to come to room temperature.

Preheat the oven to 350°F. Line baking sheets with parchment paper.

Drop spoonfuls (2-3 tablespoons) of dough onto baking sheets about 2 inches apart.

Bake for 12 minutes. The cookies are done when the sides are lightly browned and the centers still look soft.