Place egg whites, sugar, and cream of tartar in the heatproof bowl of your stand mixer. Set over a pot with simmering water to create a double broiler (make sure the bottom of the bowl does not touch the water). Whisk constantly until sugar is dissolved (3 to 4 minutes). The temp should reach 160 degrees on a candy thermometer or if you quickly dip the tip of two fingers in then rub them together you shouldn't feel any sugar granules (be careful it is hot).
Transfer bowl to stand mixer fitted with the whisk attachment and beat (starting on low speed then gradually increasing to high) until stiff, glossy peaks form. This should take about 5-7 minutes.
Add vanilla and mix until combined. Use immediately.