Timing is key, so have everything you need ready, and start whisking egg whites once the syrup is at a full boil.
Ingredients
Preparation
In a small, heavy saucepan over medium heat, combine 1 cup sugar, the corn syrup, and the water; bring to a boil, washing down sides of the pan with a pastry brush dipped in water. Raise heat to medium-high. Cook without stirring until a candy thermometer reaches 230°F. Meanwhile, with an electric mixer, beat egg whites on medium speed until soft peaks form. Gradually add remaining tablespoon sugar, and beat until peaks are very fluffy but still soft. When the syrup reaches 230°F, reduce mixer speed to medium-low; pour syrup in a steady stream down the sides of the bowl. Increase speed to medium-high, and beat until cool, about 7 minutes. Add vanilla, and beat until fully incorporated. The frosting should be shiny and fluffy and hold stiff peaks. Use immediately.
