This has become one of my favorite icing recipes for sugar cookies, especially at Christmastime. While it doesn't have the rich, buttery flavor of buttercream (my mom's standby), I like how the frosting gets crisp and make the cookies totally portable and stackable. Plus I think they look really pretty with the smooth surface, especially when a few subtle sprinkles are added.
It's also perfect for piping onto gingerbread men!
Royal Icing (from cooks.com)
3 egg whites
1 lb. confectioner's sugar
1/2 t. cream of tartar
1/2 t. almond or vanilla extract
Place all ingredients in a bowl and beat on medium high for a full 10 minutes. Icing will become thick and glossy but will not increase in volume. Keep covered with a damp cloth while using, removing small portions to bowl as needed. Tint if desired. To thin, add a few drops of water. Royal icing hardens with a nice sheen.