Ingredients
1 pound (4 sticks) unsalted butter, room temperature
1 1/2 cups sugar
6 egg whites
1 teaspoon pure vanilla extract
Preparation
Beat butter with electric mixer until fluffy and pale. Transfer to small bowl.
In double boiler over simmering water, whisk sugar and egg whites until warm and sugar is dissolved, 2 to 3 minutes. Transfer to clean bowl of electric mixer; beat on high with whisk attachment until fluffy and cooled, about 10 minutes.
Reduce mixer to medium-low; add butter a scant 1/4 cup at a time, beating well after each addition. Mix in vanilla.
Switch to paddle attachment; beat on lowest speed 3 to 5 minutes. Leave at room temperature if using same day.
