Meanwhile, in small saucepan over medium heat, combine egg whites and sugar. Cook, stirring frequently, until egg whites reach 160° F. Remove from heat, transfer to electric mixer fitted with a whisk attachment. Whisk egg whites on high until white and shiny and the bowl of the electric mixer is no longer warm, about 8 to 10 minutes. Add butter, one piece at a time, until mixture is smooth and shiny. If mixture separates, continue to whisk until it comes together. Add vanilla. Reserve 1/2 cup frosting.