Swiss Meringue Buttercream
4 large egg whites
1 1/4 cups sugar
1 1/2 cups unsalted butter, (3 sticks), softened, cut into tablespoons
2 1/2 teaspoons pure vanilla extract
Put egg whites and sugar in the heatproof bowl of an electric mixer set over a pan of simmering water.
Whisking constantly, cook until sugar has dissolved and mixture is warm (about 160 degrees.
Attach bowl to a mixer fitted with the whisk attachment.
Beat egg white mixture on high speed until it forms stiff (but not dry) peaks.
Continue beating until fluffy and cooled, about 6 minutes.
Switch to the paddle attachment. With mixer on medium-low, add butter several tablespoons at a time, beating well after each addition. (If frosting appears to separate after all butter has been added, beat on medium-high speed until smooth again, 3 to 5 minutes more.)
Beat in vanilla. Reduce speed to low; beat 2 minutes to eliminate air bubbles. Stir with a rubber spatula until smooth.