Pour into bowl of electric stand mixer fitted with whisk attachment. Beat on high speed 2 to 3 minutes or until glossy peaks form and bowl has cooled to feel slightly warm. Still on high speed, slowly add 1 1/2 cups softened butter, one piece at a time, scraping side of bowl occasionally, until mixture is smooth, thick and shiny, 12 to 14 minutes. Beat in vanilla. If mixture separates, beat until smooth.