Combine butter, sugars, cocoa powder and vanilla and mix until smooth.

Stir in egg whites until incorporated.

Add flour and baking soda; mix until thoroughly combined.

Stir in most of M&M's, reserving some for top.

Cover a large jelly roll pan with aluminum foil. Spray with cooking spray and spread dough as thinly as possible (should cover all or most of a 16×12 inch pan). Press additional M&M's into dough.

Bake at 375 degrees for 14 min.

Cool on pan; break or cut into irregular pieces.