In the top of a double boiler, whisk together egg whites, corn syrup, sugar, salt, and cream of tartar. Cook over simmering water, stirring constantly, until sugar dissolves and an instant-read thermometer registers 160°F (71°C), about 7 minutes.

Carefully transfer mixture to the bowl of a stand mixer. Using the whisk attachment, beat at high speed until doubled in volume and stiff peaks form, 3 to 4 minutes. Beat in vanilla. Use immediately.