Place egg whites, sugar, and cream of tartar in the heatproof bowl of a stand mixer. Set over a saucepan with simmering water. Whisk constantly until sugar is dissolved and whites are warm to the touch about 3 to 4 minutes.
Transfer the bowl to the stand mixer fitted with the whisk attachment, and beat, starting on low speed, gradually increasing to high, until stiff, glossy peaks form about 5 to 7 minutes. Add vanilla, beat until combined, use immediately.