This recipe goes with Vanilla and Apricot Wedding Cake Recipe.
Put sugar and egg whites in a heatproof mixer bowl; set over a pan of simmering water. Heat, whisking constantly, until sugar dissolves and mixture registers 140 degrees on an instant-read thermometer.
Transfer bowl to mixer stand; whisk on medium-high until fluffy and cooled, about 10 minutes. Continue beating until stiff, glossy peaks form.
Reduce speed to medium low; add butter by the tablespoon, beating well after each. Beat in vanilla. if making apricot buttercream, add cooled apricot puree, and beat until smooth. Use immediately, or refrigerate airtight up to 1 week or freeze up to 1 month. Bring to room temperature, and beat on low speed until smooth, about 3 minutes, before using. If buttercream separates, beat until smooth again.