In the heat-proof bowl of an electric mixer, combine light-brown sugar and egg whites. Place the bowl over a pot of gently simmering water; whisk until mixture becomes hot to the touch, about 5 minutes.Advertisement
Return bowl to mixer stand. Whip mixture with the whisk attachment until cooled, about 12 minutes. Switch to paddle attachment; add butter 1 tablespoon at a time. Stir in vanilla.