1. Bring to a boil the sugar, corn syrup and water, stirring until the sugar dissolves. Continue cooking to 242° F. (syrup forms a firm ball in cold water).
2. In an electric mixer or a large bowl, beat the egg whites until stiff but not dry. Add the syrup in a fine stream while beating constantly.
3. Add the vanilla and continue beating until the frosting will hold its shape when dropped from the beater back into the bowl.
4. Spread the frosting quickly on the cake.