Marshmallow Frosting

Place egg whites, sugar, and cream of tartar in the heatproof bowl of an electric mixer. Set over a saucepan with simmering water, making sure the water is not high enough to touch the bowl. Whisk constantly until sugar is dissolved and whites are warm to the touch, 3 to 4 minutes.

Transfer bowl to electric mixer fitted with the whisk attachment, and beat, starting on low speed, gradually increasing to high, until stiff, glossy peaks form, 5 to 7 minutes. Add vanilla, and mix until combined. Use immediately.