Swiss Meringue Buttercream
In a heatproof mixing bowl set over a pan of simmering water, whisk sugar and egg whites until sugar dissolves and mixture is warm to the touch, 3 to 4 minutes. Remove from heat; beat on medium-high speed until completely cooled and stiff peaks form, about 10 minutes.Advertisement
With mixer on medium-low speed, add butter by the tablespoon, beating well after each addition. Beat in vanilla. Use immediately, or transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month. Before serving, bring to room temperature; using the paddle attachment, beat on low speed approximately 3 minutes.