To prepare frosting, beat vanilla frosting at medium speed in a mixing bowl, gradually adding 1/4 cup melted chocolate discs. Beat at high speed 30 seconds or until smooth and well blended. Fold in vanilla and whipped topping. Refrigerate. Preheat oven to 350. Prepare a 13 x 9" pan with cooking spray and flour; set aside. To prepare batter, combine cake mix, whipped topping mix, and baking powder. In another mixing bowl, combine water, egg whites, oil, and vanilla. Mix dry ingredients with wet ingredients just until moistened. Gradually beat in remaining chocolate discs until well blended. Pour batter into prepared pan. Bake for 40 minutes or until cake is light brown. Cool 10 minutes. Meanwhile, to prepare filling, over low heat, add powdered sugar, chocolate chips, margarine, and corn syrup. Cook until well blended and melted. Spread filling over warm cake. Cool completely. Frost cake. Store in refrigerator. Let stand at room temperature 10 minutes before cutting.