- 1 pound (4 sticks) unsalted butter, room temperature
- 1 1/2 cups sugar
- 6 egg whites
- 1 teaspoon pure vanilla extract
- 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.
Store airtight in refrigerator up to 3 days. Bring to room temperature; beat until smooth.