Swiss Meringue Buttercream for Tiered Cakes

In the bowl of an electric mixer fitted with the paddle attachment, beat butter until fluffy and pale. Transfer to a bowl.

Fill a medium saucepan one-quarter full with water; bring to a boil. Simmer.

Combine sugar and egg whites in heat-proof bowl or clean bowl of electric mixer; place over saucepan. Whisk constantly until sugar dissolves and whites are warm to the touch, 3 to 4 minutes. Test by rubbing between your fingers.

Beat egg-white mixture on medium speed until fluffy and cooled, about 10 minutes. On high speed, whisk until stiff peaks form. Reduce speed to medium-low; add butter a few tablespoons at a time, beating well. Stir in vanilla.

Switch to paddle attachment; beat mixture on lowest speed, 3 to 5 minutes. Leave at room temperature if using the same day.