One of the first lessons a cake decorator learns is the importance of keeping frosting covered, so it doesn't become dry, hard and useless for spreading atop a cake or cookie. Buttercream frosting is intended to harden slightly after being piped on a cake -- this helps hold the shape of flowers and borders -- but not before it meets the cake. When frosting becomes too hardened to handle, look for ways to return it to its original smooth texture.
Stir the frosting well to break up the dry, hardened crust on the surface. Use an electric mixer or wire whisk to thoroughly mix and break up hard chunks. If only a thin layer of frosting on the top has become dry, mixing might be enough as you distribute the moisture from the rest of the frosting.
Mix a few drops of milk or water -- whichever liquid you originally used to make the frosting -- to rehydrate hardened frosting. For a 1-cup portion of frosting, start with a few drops; mix in a teaspoon at a time if you have a large bowl of hardened frosting. Add more liquid and stir as needed until you achieve the original consistency.
Heat the frosting in the microwave for about 10 seconds and mix well. Repeat this process, heating the frosting five to 10 seconds at a time and mixing in between until you achieve the original smooth consistency. Use this method sparingly and as a last option. This melts and softens the frosting, but microwaving for too much time can lead to a runny, syrupy mess.