In medium bowl, beat egg whites with electric mixer on high speed just until stiff peaks form.
In 1-quart saucepan, stir sugar, corn syrup and water until well mixed. Cover and heat to rolling boil over medium heat. Uncover and boil 4 to 8 minutes, without stirring, to 242°F on candy thermometer or until small amount of mixture dropped into cup of very cold water forms a firm ball that holds its shape until pressed. For an accurate temperature reading, tilt the saucepan slightly so mixture is deep enough for thermometer.
Pour hot syrup very slowly in thin stream into egg whites, beating constantly on medium speed. Add vanilla. Beat on high speed about 10 minutes or until stiff peaks form. Frosts 13x9-inch cake generously, or fills and frosts an 8- or 9-inch two-layer cake.