Place the egg whites and sugar into a metal bowl and set over a pan filled with about 2 inches of simmering water. Heat, stirring frequently, until the temperature of the egg whites reaches 140 degrees. They will feel very hot to the touch.
Transfer the heated egg whites and sugar to a large mixing bowl or stand mixer. Mix at high speed (or as high as you can go without egg flying out of the bowl) until they have reached their maximum volume, 5 to 10 minutes. Mix on medium or medium-high speed while pinching off small pieces of butter and throwing them in. Mix in vanilla if using. The buttercream may look like it is breaking down, but keep on mixing and it will come back together.