1. Heat oven to 350 degrees. Line 13x9-in. pan with foil, with ends of foil extending over sides; spray with cooking spray. Beat yellow cake mix, whipped topping and egg in large bowl with mixer until blended. (Batter will be stiff.)
2. Chop chocolate and add to batter along with walnuts; mix well. Spread onto bottom of prepared pan. Bake 23 to 25 minutes or until toothpick inserted in center comes out clean. Cool completely. Use foil handles to remove dessert from pan before cutting into bars.