Swiss Meringue Buttercream
Place the egg whites and sugar in large metal mixer bowl set over simmering water. Whisk constantly until the sugar melts and the mixture is very thin and warm.
Remove the bowl from the heat and whisk on high speed until stiff peaks form, about 5 minutes. Continue beating on low speed until cool, about 15 more minutes.
Beat in small pieces of the cool but soft butter on low speed. The mixture may "curdle" before coming together. Add vanilla extract or other flavorings if desired. Rebeat occasionally while frosting the cake to maintain a smooth texture.