This icing dries very quickly, so keep it covered at all times with a damp cloth to help keep it moist.
Combine egg whites, 1 cup powdered sugar, and cream of tartar in top of a double boiler. Place over simmering water. Cook, stirring constantly with a wire whisk, 9 minutes or until mixture reaches 160°. Remove from heat. Transfer to a large mixing bowl, and add remaining powdered sugar.
Beat at high speed of an electric mixer 5 to 8 minutes or until stiff peaks form.