Preheat oven to 350°. Spray a 9 x 13-inch pan with cooking spray. In large saucepan, cook pudding and milk according to package directions. Blend dry cake mix thoroughly into hot pudding, beating by hand or with mixer for 1 to 2 minutes. Pour into prepared pan. Sprinkle with chocolate pieces and nuts. Bake 30 to 35 minutes, or until wooden toothpick inserted in center comes out clean. Serve warm or cool. Top with whipped topping, if desired.