How to Make It
In a double boiler, beat the egg whites with the sugar over simmering water until the sugar has dissolved and the whites reach 160° on a candy thermometer, about 7 minutes.
Pour the egg whites into the bowl of a standing electric mixer fitted with the whisk and beat at medium speed until stiff, glossy and cool. Beat in the vanilla. At medium-low speed, beat in the butter, a few tablespoons at a time; continue beating until the buttercream is fluffy yet firm.