Bake your favorite 2-layer cake recipe or mix in 8 in. round pan following recipe instructions. Cool in pan on cooling grid 15 minutes; remove from pan and cool completely. Stack layers on foil-wrapped cake board.
Tint buttercream icing pale green. Spatula ice cake. Thin white buttercream with corn syrup; pipe tip 2 sotas to cake. Add tip 21 green shell bottom border; attach icing decorations to border. Pipe a small tip 21 mound of green buttercream to top of cake; attach icing decorations.