Bake and cool a 2-layer round cake. Stack cooled cake layers using buttercream icing or another favorite filling between layers. Place on a foil-wrapped cake circle.
Position cake on turntable to decorate. Spatula ice cake smooth with yellow buttercream icing; ice sides at least 1/2-inch thick. Using small-tooth edge of Decorating Comb, comb cake sides to form ridges.
Cover cake top with 2 layers of tip 1M rosettes. Add tip 1M bottom border rosettes and tip 18 rosettes to cake sides. Sprinkle rainbow jimmies over cake top and bottom border. Position candles on cake top.