Swiss Meringue Buttercream for Marble Cake
Put egg whites and sugar in the heatproof bowl of an electric mixer set over a pan of simmering water. Whisk constantly until sugar is dissolved and mixture registers 140 degrees on an instant-read thermometer, 2 to 3 minutes.Advertisement
Transfer bowl to mixer; fit mixer with the whisk. Beat on high until mixture is fluffy and cooled, about 10 minutes.
Reduce speed to medium-low; add butter several tablespoons at a time, mixing well after each addition. Add vanilla, and mix until buttercream comes together, 3 to 5 minutes. Reduce speed to low, and mix until buttercream is smooth and no air bubbles remain, about 2 minutes.