Bake and cool cake. Prepare batter following recipe directions. Place bakeware on cookie sheet. Bake and cool cakes following package directions.
Decorate cake. Prepare buttercream icing following recipe directions. Use red-red icing color to tint portion of icing. Reserve small amount of white icing. Use 9 in. spatula and red icing to ice top of cake smooth.
Use rose icing color to tint remaining white icing pink. Use tip 3 and pink icing to pipe diagonal stripes, 3/8 in. apart, across top of cake.