Preheat oven to 350. Prepare two 8" cake pans with cooking spray and flour; set aside. In a mixing bowl, combine cake mix, whipped topping mix, and baking powder. In another mixing bowl, combine water, egg whites, oil, and vanilla extract. Mix dry ingredients with wet ingredients just until moistened. Pour evenly into prepared pans. Bake for 25 minutes or until golden brown. Freeze one layer for future use. Prepare pudding mix according to package directions. Cool. Split cooled cake layer. Spread bottom with pudding mix. Place top of cake layer over the pudding layer. To prepare icing, combine powdered sugar, cocoa powder, salt, margarine, and water until smooth. Frost top with icing, and let additional run down sides of cake.