Classic Seven-Minute Frosting

Heat 1 1/2 cups sugar, the water, and corn syrup in a small saucepan over medium heat, stirring occasionally, until sugar dissolves. Raise heat to medium-high, and bring to a boil. Cook, washing down sides of pan with a wet pastry brush to prevent sugar crystals from forming, until syrup reaches 230 degrees on a candy thermometer. Once syrup reaches 230 degrees, remove from heat immediately.

Meanwhile, with a mixer on medium speed, whisk egg whites to soft peaks. Whisk in remaining 1/4 cup sugar. Reduce speed to medium-low, and pour syrup into egg whites in a slow, steady stream down the side of the bowl. Raise speed to medium-high, and whisk for 6 minutes. Add vanilla; whisk until soft peaks form, about 1 minute. Use immediately.