Combine the egg whites with the sugar in a mixing bowl.
Place the bowl over simmering water, and heat until the thermometer's temperature reaches 140° F.
To avoid cooking the egg whites, continually whisk the mixture, throughout.
Remove from the heat and using the whisk attachment, whip the mixture at high speed until completely cooled.
Lower the speed of your mixer. Add your vanilla.Gradually add the butter in small chunks.
The buttercream will break momentarily, but will emulsify as you continue to whip and add more butter.