Make-Ahead Swiss Meringue Buttercream
Beat butter with electric mixer until fluffy and pale. Transfer to small bowl.Advertisement
In double boiler over simmering water, whisk sugar and egg whites until warm and sugar is dissolved, 2 to 3 minutes. Transfer to clean bowl of electric mixer; beat on high with whisk attachment until fluffy and cooled, about 10 minutes.
Reduce mixer to medium-low; add butter a scant 1/4 cup at a time, beating well after each addition. Mix in vanilla.
Switch to paddle attachment; beat on lowest speed 3 to 5 minutes. Leave at room temperature if using same day.
Store airtight in refrigerator up to 3 days. Bring to room temperature; beat until smooth.