1) In a small saucepan, sugar, corn syrup, water, and salt.
2) Cover and heat to boil.
3) Remove the lid, and allow to boil gently until the temperature comes to 242° on a candy thermometer.
4) In a large bowl, beat egg whites and cream of tartar to stiff peaks.
5) Gently stream hot syrup in the egg whites mixture, beating continuously at high speed until frosting is stiff.
6) Fold in the vanilla.
7) Spread over cake.
If there is no thermometer available heat the sugar mixture falls threadlike from the spoon.