In the bowl of a stand mixer combine egg whites and sugar. Place the bowl over a pot of simmering water and whisk constantly until the mixture reaches 160 degrees (F).
Place the bowl on the stand mixer fitted with a whisk attachment and whip on medium-high speed until it has doubled in volume and reaches still peaks.
Begin adding the butter slowly, about 1 every 10 seconds, until all of the butter is fully combined. Add the vanilla.
Turn mixer to high and beat until the buttercream is light and fluffy.
At this point, you can food coloring or other flavorings