Preheat oven to 350.
Spray 13 x 9 inch pan with cooking spray; set aside.

Frosting:
Beat vanilla frosting at medium speed in a mixing bowl, gradually adding 1/4 cup melted chocolate.
Beat at high speed until smooth and well blended.
Fold in vanilla and whipped topping.
Refrigerate.

Cake:
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 just until moistened.
Gradually beat in remaining chocolate until well blended.
Pour batter into prepared pan.
Bake 40 minutes or until cake is light brown.
Cool 10 minutes.

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.