Once piped or spread onto your cake, cupcakes, or cookies, this buttercream will crust up. If you want to add sprinkles or other decorations, make sure to do that right after piping it on, before it has a chance to form a crust. Even though it crusts up, it won't completely harden. Be careful if storing or transporting baked goods decorated with this frosting so that you don't smash your beautiful decorating work.