White Mountain Frosting

Grandma New's recipe.  

Preparation

Mix sugar, corn syrup and water in saucepan.  Cover; heat to rolling boil over medium heat.  Remove cover and boil rapidly to 242 degrees on candy thermometer or until small amount of mixture dropped into very cold water forms a firm ball that holds its shape until pressed.  As mixture boils, beat egg whites until stiff peaks form.  Pour hot syrup very slowly in a thin stream into egg whites, beating constantly on medium speed.  Add vanilla; beat on high speed until stiff peaks form.