To decorate a cake with dots or an intricate design, first cover it with a smooth base of brown-sugar buttercream. Be sure to use room-temperature butter in this recipe. If the butter is too cold, the buttercream will appear curdled.
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.
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.