Easy Swiss Meringue Buttercream
Put the sugar and egg whites in a clean heatproof mixer bowl, and set over a pan of simmering water. Whisk constantly until the sugar is dissolved and the mixture registers 140 degrees on an instant-read thermometer.Advertisement
Transfer the bowl to the mixer stand, and whisk on medium-high speed until fluffy and cooled, about 10 minutes. Continue beating until stiff, glossy peaks form.
Reduce the speed to medium-low; add the butter by the tablespoon, beating well after each addition. Beat in the vanilla. The buttercream can be refrigerated in an airtight container for up to 1 week or frozen for up to 1 month. Bring to room temperature before using; beat on low speed until smooth, about 3 minutes.
Amounts for filling and frosting: The amounts provided are for a standard two-layer 4-inch-tall tier, filled with a 1/4-inch-thick coating of buttercream between the layers. For 6-inch: 3/4 cup to fill, 1 1/2 cups to frost; 8-inch: 1 1/4 cups to fill, 2 cups to frost; 10-inch: 2 1/2 cups to fill, 3 1/2 cups to frost; 12-inch: 3 cups to fill, 4 1/4 cups to frost. (Note: Many cake makers will split the two cake layers in half horizontally and spread filling between all four resulting layers; if this is your plan, triple the filling amount.)