Combine sugar and egg whites in mixer bowl, and set over a pan of simmering water. Whisk constantly until sugar is dissolved and mixture registers 140 degrees on an instant-read thermometer.
Transfer bowl to mixer; whisk on medium-high speed 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 addition. Whisk in vanilla.
Buttercream can be refrigerated in an airtight container up to 3 days or frozen up to 1 month. Bring to room temperature before using; beat on lowest speed until smooth, about 3 minutes.
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