Place sugar, cream of tartar or corn syrup, salt, water, and egg whites in the top of a double boiler. Beat with a handheld electric mixer for 1 minute. Place pan over boiling water, being sure that boiling water does not touch the bottom of the top pan. (If this happens, it could cause your frosting to become grainy). Beat constantly on high speed with electric mixer for 7 minutes. Beat in vanilla.