Buttercream icing is a favourite with cakes of every flavour. It's also a popular choice for decorating. Because buttercream icing starts out so soft, an important part of using it for cake decorating is allowing it to "crust" or harden slightly on the surface. This makes the surface of the cake smoother and better for decorating, and crusting allows the decorations such as roses or piped lines to stay where they are supposed to while still being soft enough to cut.
Spread the buttercream icing on the cake until it covers the entire surface.
Smooth the surface of the icing with a spatula or the edge of a smooth knife.
Allow the icing to sit undisturbed for approximately 10 minutes. Test the crust by touching your finger to the icing gently. If it comes away sticky, allow it to sit until your finger comes away clean when you touch it.
Decorate the cake as you wish with the additional icing once the surface has set. Allow the same time frame for any additional decorations you add to the cake.
Keep a damp paper towel over the bowl of icing to keep it from hardening.
If the icing in the bowl crusts before you are ready, do not stir it or it will make the icing gritty. Instead, carefully scrape off the top, hardened layer and then stir it.