Use this recipe when making our Block Cakes and Bootie Cakes.
In a heatproof mixing bowl set over a pot of simmering water, whisk together the sugar and egg whites until sugar has dissolved and mixture is warm, 2 to 3 minutes. Remove bowl from heat.
Using an electric mixer, beat egg-white mixture on high speed until fluffy and completely cooled, about 10 minutes.
With mixer on low speed, add the butter, a tablespoon at a time, beating until fully incorporated after each addition. (Don't worry if your buttercream appears curdled after all the butter has been added; continue beating until smooth again.)
Add the vanilla extract, and beat until combined. If using buttercream the same day, let stand at room temperature, covered tightly with plastic wrap. When tinting buttercream, add food coloring a drop at a time, and stir well before adding more.
Transfer buttercream to an airtight container, and refrigerate up to 5 days; before using, bring to room temperature, and beat until smooth, about 10 minutes. Do not tint before storing, as color tends to fade.