Meringue Buttercream for Butter Cakes

Whisk sugar and egg whites in a large heatproof bowl set over a pan of simmering water until sugar is dissolved and mixture registers 140 degrees on an instant-read thermometer, 2 to 3 minutes.

Fit an electric mixer with the whisk attachment, and beat the egg-white mixture on high speed until it holds stiff (but not dry) peaks and mixture is fluffy and cooled, about 10 minutes.

Reduce speed to medium-low, and add butter several tablespoons at a time, beating well after each addition (meringue will deflate slightly as butter is added). Add vanilla, beat until frosting comes together, 3 to 5 minutes. Beat on the lowest speed until air bubbles diminish, about 2 minutes. Stir with a rubber spatula until frosting is smooth.