In the bowl for an electric mixer beat together butter, sugar, and brown sugar until fluffy (about 2 minutes).
Add in the eggs, vanilla, peppermint extract, and melted chocolate. Mix until well combined, scraping the sides as necessary.
In a small bowl whisk together flour, cocoa powder, baking powder and salt until combined. Slowly add flour mixture to chocolate mixture until combined.
Using a spatula fold in the M&M's. Cover dough, and refrigerate for at least 2 hours, up to 36 hours.
Preheat oven to 350º F.
Scoop balls of dough onto a baking sheet. Top with a few extra M&M's.
Bake for about 10 minutes, until the edges are set and the tops appear completely dry.
Remove from oven, and cook for a few minutes before removing to a wire rack.