In the top of a double boiler combine sugar, cold water, egg whites, and cream of tartar or corn syrup. Beat with an electric mixer on low speed for 30 seconds.
Place over boiling water (upper pan should not touch water). Cook, beating constantly with the electric mixer on high speed, about 7 minutes or until frosting forms stiff peaks. Remove from the heat; add vanilla. Beat 2 to 3 minutes more or until frosting reaches spreading consistency. This frosts the tops and sides of two 8- or 9-inch cake layers or one 10-inch tube cake. Store frosted cake in the refrigerator and serve the same day it is made. Makes 4 cups.