Use this buttercream to frost our Citrus Wedding Cake.
In electric mixer with paddle attachment, beat butter until fluffy and pale.
Combine sugar and egg whites in clean, heat-proof mixer bowl; place bowl over pan of simmering water. Whisk until sugar dissolves and whites are warm to the touch, 3 to 4 minutes.
Transfer bowl to mixer; with whisk attachment beat on medium-high until fluffy and cool, and stiff peaks form, about 10 minutes. Reduce speed to medium-low; add butter a few tablespoons at a time, beating well after each. Add vanilla bean scrapings and extract.
Switch to paddle; beat on lowest speed to remove air bubbles, 3 to 5 minutes. Store in airtight container in refrigerator up to 3 days. Before using, bring to room temperature and beat until smooth.