Preheat oven to 350 degrees. Line a 11 3/4" x 7 3/4" baking pan with foil and spray with cooking spray.
Mix cake mix, egg, butter and vanilla until blended. Fold in half of the M & Ms. Press batter into the prepared pan and sprinkle with the remaining M & Ms, pressing them into the batter. Bake for 35-40 minutes or until inserted toothpick comes out clean. Allow to completely cool before cutting into bars.
Red Velvet Cake Mix Bars https://www.yellowblissroad.com/red-velvet-cake-mix-bars/