This recipe is used in our Scrumptious S'Mores Cake.
Place egg whites, sugar, and cream of tartar in a heatproof mixer bowl set over a pan of simmering water. Whisk until sugar dissolves and mixture registers 160 degrees on a candy thermometer. Attach the bowl to an electric mixer fitted with a whisk attachment and whisk on high speed until stiff, glossy peaks form and mixture is cool, about 10 minutes. Whisk in vanilla. Use immediately.