Begin by beating the egg whites in the bowl of a standing mixer using the whisk attachment until stiff peaks form. You can determine if the peaks are "stiff" by dipping the whisk into the mixture and pulling upward. If the egg whites hold their peak, they are ready. If they melt back into the bowl, whisk longer.
Meanwhile, in a small saucepan, heat the sugar, corn syrup, and water over medium heat. Stir well and cover with a lid until the mixture comes to a boil. Remove the lid and boil without stirring for about 5-7 minutes, or until the mixture reaches 242 degrees on a candy thermometer (firm ball).
With the mixer turned on to medium, begin pouring the hot sugar mixture into the egg whites in a slow stream.
Add the vanilla.
Continue beating for 10 minutes on medium speed or until stiff peaks form once again.
Spread over a 13x9 inch cake, frost and fill an 8-inch round cake, or pipe onto cupcakes.