In a large mixing bowl, mix together butter and sugar. Add eggs, one at a time, beating well after each one. Mix in vanilla.
In a separate bowl, combine the flour, cocoa, baking soda and salt; gradually add to the wet ingredients, and stir until everything is combined and has a cookie dough. Stir in the mini M&Ms.
Cover and refrigerate for about 1 hour or until easy to handle.
Roll dough into 1-in. balls. Place each one 2 in. apart on a baking sheets. Add additional mini M&Ms to the top of each cookie, if desired.
Bake at 350 degrees F for 8-10 minutes. Cool on wire racks.