In a stand mixer, combine melted butter, brown sugar, granulated sugar, and vanilla. Mix on low until combined.
Add eggs one at a time, followed by the dry ingredients, while the mixer is on low.
Use a rubber spatula to fold in M&Ms and chocolate chips. Cover the bowl with clear wrap and chill in the refrigerator for 30 minutes.
Preheat the oven to 350°F and line a cookie sheet with parchment paper.
After chilling, use a large cookie scoop (about 2 1/2-3 tablespoons of dough per cookie) and place onto the cookie pan, spacing about 2 inches apart.
Bake for 10-12 minutes, or until the edges are just set.
As soon as the cookies come out, use a round cookie cutter or glass to gently reshape them into a circular shape.
Let cool for 4 minutes on the baking sheet before transferring to a cooling rack. Place cookies in the freezer for 15 minutes to chill.