Royal icing is a low-fat icing typically made with powdered sugar to which egg whites of meringue powder is added. The egg whites or the egg whites in the meringue powder cause the icing to set up with a hard shell. The more the egg white there is in the frosting, the harder the shell. Royal icing is usually thought of as a decorating icing but a hard shell is useful on cookies to keep the frosting from marring in handling or shipping.
If you are reluctant to use raw eggs in your frostings, then meringue powder was meant for you.
The following recipe will make an icing that dries with a medium shell. (Different meringue powders will perform differently. You may need to experiment with your icing.)
If you choose to use egg whites, you will need the egg whites from two or three eggs depending on how hard you want the shell to be.
1 pound powdered sugar
1/8 teaspoon cream of tartar
3 tablespoons meringue powder
1 teaspoon vanilla
water as needed
1. In a medium bowl, mix the powdered sugar with the cream of tartar and meringue powder until well combined.
2. Beat in the vanilla and enough water to reach the right consistency.
Royal icing can be stored in the refrigerator with a tight lid or with plastic wrap pressed against the surface of the icing.