In the top of a double boiler combine sugar, water, egg whites, and cream of tartar (or corn syrup). Beat with an electric mixer on low speed for 30 seconds. Place the upper pan over gently boiling water (upper pan should be near but not touching water). Cook, beating constantly with the electric mixer on high speed, about 7 minutes or until stiff peaks form (tips stand straight). Remove from heat. Add vanilla. Beat frosting for 2 or 3 minutes more or until the frosting is of spreading consistency.