Combine sugar, corn syrup, and water in small saucepan.
Cover; heat to rolling boil over medium heat.
Remove cover and boil rapidly, without stirring, to 242 F on candy thermometer.
As mixture boils, beat egg whites until stiff peaks form.
Pour hot syrup very slowly in a thin stream into the beaten egg whites, beating constantly on medium speed.
Beat on high speed until stiff peaks form.
Add vanilla during last minute of beating.
VARIATIONS: Add 1/4 cup sifted cocoa for Cocoa Frosting, folding in till blended; for Lemon Frosting substitute 1 TBSP lemon juice for the vanilla and add 1/4 tsp. grated lemon peel and 10 drops of yellow food coloring; for Pink Mountain Frosting, substitute maraschino cherry juice for the water; for Satiny Beige Frosting substitute brown sugar (packed) for the granulated sugar and decrease vanilla to 1/2 tsp.