- 2 1/4 cups sugar
- 1/2 cup heavy cream
- 1 1/2 pounds (6 sticks) unsalted butter, softened
- 9 large egg whites
- 1 1/2 teaspoons pure vanilla extract
- Make caramel: Combine 1 1/4 cups sugar and 1/2 cup water in medium saucepan; bring to boil over medium-high heat. Brush down sides with damp pastry brush to prevent crystallization. Continue cooking, without stirring, until dark amber color. Remove from heat; slowly stir in cream with wooden spoon until completely smooth; set aside until cool to touch.
- In an electric mixer fitted with the paddle attachment, beat butter until fluffy and pale. Transfer to a medium bowl.
- Whisk together remaining 1 cup sugar and egg whites in clean bowl of mixer. Set over pot of simmering water; whisk gently until completely dissolved, about 3 minutes.
- Transfer bowl to mixer stand. Using whisk attachment, beat on medium speed until fluffy and cooled, about 10 minutes. Increase speed to high; whisk until stiff peaks form. Reduce speed to medium-low; add butter 1/4 cup at a time, beating well after each addition. Whisk in vanilla.
- Switch to paddle. Add caramel; beat on lowest speed 3 to 5 minutes. Leave at room temperature if using same day.
Refrigerate in airtight container up to 3 days. Bring to room temperature; beat until smooth.