- 1 1/4 cups sugar
- 5 large egg whites
- 2 cups (4 sticks) unsalted butter, room temperature
- 1 teaspoon pure vanilla extract
In the heatproof bowl of an electric mixer, combine sugar and egg whites. Place bowl over pan of simmering water; whisk until whites are hot and sugar is dissolved, 2 to 3 minutes.
Remove from heat, and transfer to mixer fitted with the whisk attachment. Beat on high speed until cooled and fluffy, 10 to 15 minutes. Reduce speed to medium. Add butter, 1 tablespoon at a time. Beat until incorporated. Stir in vanilla.