1. Fill a medium saucepan one-quarter full with water, and bring to a boil. Reduce to a simmer.

2. Combine egg whites, sugar, and cream of tartar in a heat-proof bowl or bowl of electric mixer; place over saucepan. Whisk constantly until sugar is dissolved and whites are warm to the touch, 3 to 4 minutes. Test by rubbing between your fingers. Transfer bowl to an electric mixer fitted with whisk attachment; whisk on low speed, gradually increasing to high speed, until stiff, glossy peaks form, about 10 minutes. Add vanilla; mix until combined. Use immediately.