A crusting buttercream frosting forms a crust as the surface dries, which allows you to create a smooth surface for applying decorative piping without smudging the frosting. Vegetable shortening is substituted for all or part of the butter in a traditional buttercream recipe because shortening solidifies more than butter and will not melt as easily with temperature changes. While you can smooth the surface of crusting buttercream tediously using a spatula warmed in hot water to soften the frosting, it's much easier and faster to use a smooth paper towel or wax paper.
Frost the cake with the crusting buttercream frosting and spread it as smooth as possible. If the frosting stiffens and becomes difficult to spread, dip your icing spatula in a glass of hot water to heat the blade, wipe the water on a towel and spread the icing with the heated spatula. The heat softens and slightly melts the hardening sugar and fat in the frosting so it spreads more easily. Don't spend too much time trying to make it perfect because you can add the finishing touches later.
Leave the cake uncovered and allow it to sit at room temperature or in the refrigerator for 15 to 30 minutes or until the top of the frosting feels hard to the touch. This can take up to about 30 minutes in areas with high humidity. You don't want the frosting to harden too much. When you touch your finger to the cake and don't end up with wet frosting on your finger, it's ready for smoothing.
Lay a smooth paper towel -- without a dimpled texture -- or a piece of wax paper on top of the cake. Run your hand across the paper towel or wax paper several times while applying light pressure to press down any ridges in the buttercream. Sweep forward, backward and side to side with long, straight strokes. You can also run a fondant rolling pin or fondant smoothing tool, which looks like a plastic clothes iron, over the paper towel or paper.
Reposition the paper towel or wax paper until the entire top of the cake is smooth. Reposition it along the sides of the cake and repeat the process. It helps to curve your hand slightly to conform to the sides of round cakes.
Set the towel or paper so it drapes over the edge of the cake and use your hand or a fondant smoother to smooth out the edge where the top and sides meet. Skip this step if you prefer a sharp rather than rounded edge.