6 large egg whites, at room temperature
1 cup sugar
1/4 teaspoon salt
4 sticks unsalted butter, cut into small pieces, at room temperature
2 teaspoons pure vanilla extract
Chocolate Swiss Buttercream: Make Swiss Buttercream, using just 1 teaspoon vanilla. After adding the vanilla, switch to the paddle attachment and beat in 8 ounces cooled melted semisweet chocolate until smooth and fluffy.