Preheat oven to 350 degrees F. Line baking sheets with parchment or silicone baking mats.

In a large bowl cream butter and sugars until combined. Add eggs, one at a time, beating well after each addition. Add vanilla extract and beat to incorporate. 

In a separate bowl sift flour, cocoa powder, baking soda and salt together. Slowly add to butter mixture, stirring until well combined.

Fold in mini M&M's. Chill dough at least one hour and up to 24 hours.

Drop dough by rounded tablespoons onto prepared baking sheets, about 2 inches apart.

Bake 7 - 10 minutes. Remove from cookie sheet to wire rack to cool completely. Store in a tightly covered container.