Fluffy and delicious, this vanilla Swiss meringue buttercream is a versatile topping.
Whisk egg whites and sugar in the heatproof bowl of an electric mixer set over a pan of simmering water until sugar has dissolved and mixture registers 160 degrees on an instant-read thermometer, 2 to 3 minutes.
Attach bowl to mixer fitted with the whisk attachment. Beat on high speed until fluffy and cooled, about 10 minutes. Reduce speed to medium-low; add butter, several pieces at a time, beating well after each addition. Beat in vanilla seeds. Reduce speed to low; beat until smooth.