Preheat oven to 350°.
To prepare cake, combine first 4 ingredients in a large bowl, and beat at low speed of a mixer until moist. Beat mixture at medium speed 2 minutes; pour into a 13 x 9-inch baking pan coated with cooking spray. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool cake completely in pan on a wire rack.
To prepare topping, combine 1/2 cup milk, granulated sugar, and marshmallows in a medium saucepan; cook over medium heat 5 minutes or until marshmallows are melted. Stir in coconut and cornstarch; bring to a boil. Cook 1 minute, stirring constantly. Spread topping evenly over cake; cool.
To prepare frosting, combine powdered sugar, cocoa, 2 tablespoons milk, and margarine in a small saucepan; bring to a boil over medium-high heat. Remove mixture from heat; add chocolate chips and corn syrup, stirring until chips melt. Spread frosting gently over coconut layer. Cool until set.