Cream together butter and sugars until light and fluffy.
Beat in eggs and vanilla.
In a bowl, whisk together flour, cocoa powder, milk powder, baking soda and salt. Add to butter mixture and beat until combined.
Fold in M&M's (I like to reserve some to place on the top of cookies before I bake them.) Cover dough and chill for at least 1 hour but up to several days.
Scoop in 3 Tablespoon scoops (a large cookie scoop) onto baking sheet, leaving cookies 2 inches apart.
Bake for 10-12 minutes, they should look a bit soft still. Let cool for 5 minutes on the cookie sheet and then move to a wire rack to cool completely.
Notes
Store cookies in an airtight container at room temperature for up to a week. Freeze for longer storage.