1. Heat oven to 350°F. Grease 13x9x2-inch baking pan. Prepare cake according to package directions. Stir in mashed banana.
2. Bake 30 to 35 minutes or until wooden pick inserted in center comes out clean. Poke holes with end of wooden spoon about 1 inch apart into warm cake (do not pierce bottom of cake).
3. Place chocolate spread in small microwave-safe bowl. Microwave at MEDIUM (50%) 45 seconds; stir. If necessary, microwave an additional 10 seconds at a time, stirring after each heating, just until spread is smooth and very fluid when stirred. Spread evenly over cake surface. Cool completely in pan on wire rack.
4. Frost with ONE-BOWL BUTTERCREAM FROSTING. 12 to 16 servings.
ONE-BOWL BUTTERCREAM FROSTING
6 tablespoons butter or margarine, softened
1/2 cup HERSHEY'S Cocoa
2-2/3 cups powdered sugar
1/3 cup milk
1 teaspoon vanilla extract
Beat butter in medium bowl. Add cocoa and powdered sugar alternately with milk; beat to spreading consistency (additional milk may be needed). Blend in vanilla. About 2 cups frosting.