Using an electric mixer, beat butter, sugar, eggs, and vanilla until light and fluffy.
Whisk together flour, cocoa powder, baking soda, and salt.
Add flour mxture and mix just until incorporated.
Mix in chocolate chips and m&ms. Cover and refrigerate for 60 minutes.
Preheat oven to 350 degrees and line two baking sheets with parchment paper.
Use a small ice cream scoop to drop batter onto baking sheets. Or use a spoon to scoop tablespoon-sized amounts of batter.
Bake for 8 to 10 minutes. Press more m&ms into tops of cookies when they come out of the oven if desired.