Combine sugar and egg whites in a heatproof mixer bowl set over a pan of simmering water. Whisk constantly until sugar is dissolved and mixture registers 160 degrees on an instant-read thermometer.
Transfer bowl to mixer stand; whisk on medium-high until whites are fluffy and cooled, about 10 minutes. Continue to whisk until stiff, glossy peaks form. Reduce speed to medium-low; add butter by the tablespoon, whisking well after each. Whisk in vanilla. Use immediately, or refrigerate airtight up to 3 days, or freeze up to 1 month. Bring to room temperature before using; beat on low until smooth, about 3 minutes.
Adorn your wedding cake with one of these thoughtful handmade touches.See the Ideas