This frosting hardens quickly, so prepare it just before assembling the cake.
- 1 3/4 cups sugar
- 2 tablespoons light corn syrup
- 6 large egg whites room temperature
- 1 teaspoon pure vanilla extract
Cook 1 1/2 cups sugar, 1/4 cup water, and the corn syrup in a small saucepan over medium heat, stirring occasionally, until sugar is dissolved, about 4 minutes. Raise heat to medium-high; cook, stirring, washing down sides of pan occasionally with a pastry brush dipped in water to prevent crystals from forming, until mixture comes to a boil. Cook, without stirring, until mixture registers 230 degrees on a candy thermometer, 4 to 8 minutes depending on humidity. Remove from heat.
Meanwhile, beat egg whites on medium speed in the bowl of an electric mixer fitted with the whisk until soft peaks form, 2 to 3 minutes. With mixer running, add remaining 1/4 cup sugar in a slow, steady stream. Reduce speed to medium-low, and carefully pour hot syrup down side of bowl.
Beat on medium until frosting is thick, shiny, and completely cooled, about 7 minutes. Mix in vanilla. Use immediately.