Use this buttercream for our Brown-Sugar Marble Wedding Cake.
Whisk sugar and whites in heatproof mixer bowl over simmering water until hot, 3 to 5 minutes. Return bowl to mixer. Whisk until cooled, about 12 minutes. Switch to paddle; add butter, 1 tablespoon at a time. Stir in vanilla. Use within 4 hours, or refrigerate up to 5 days.