1. Beat egg whites and cream of tartar with an electric mixer at high speed until foamy in a small bowl.
2. Beat in 10X sugar gradually, until frosting stands in firm peaks and is stiff enough to hold a sharp line when cut through with a knife.
(Tip: To keep frosting from drying out while you are decorating your cake, cover the bowl with damp paper toweling.) Store any left over in tightly covered jar in refrigerator.