How to Make It
In a bowl, using a wooden spoon, cream the butter with the vanilla.
In a large heatproof bowl, whisk the sugar with the egg whites. Set the bowl over a pan containing 1 inch of simmering water and whisk the egg whites constantly until the sugar is dissolved. Remove the bowl from the heat. Using an electric mixer, beat the whites at high speed until a medium-firm meringue forms, about 5 minutes.
Reduce the speed to moderate. Beat the creamed butter into the meringue 1/4 cup at a time, beating until it is fully incorporated before adding more. Use immediately or refrigerate.