Preheat oven to 325 degrees. Spray a 9 1/2 in. pie plate with non-stick cooking spray.
Cream together butter and brown sugar in a large mixing bowl. Add egg and vanilla and mix until combined. Combine flour and baking soda in a separate bowl. Add to batter and mix until completely mixed in. Fold in M&M's. Pour batter into pie plate and spread out evenly.
Bake 25-30 minutes or until golden brown. Let cool and cut into pieces.