Combine butter, brown sugar, granulated sugar and vanilla and mix until smooth.
Add flour and baking soda; mix until thoroughly combined.
Stir in M&M's.
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 16x12 inch pan).
Bake at 375 degrees for 12 min.
Cool on pan; break or cut into irregular pieces.