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

Remove bowl from heat, and place on a kitchen towel-lined counter. Beat with a hand mixer fitted with the whisk attachment at high speed until cool to the touch, doubled in volume, and stiff peaks form, 6 to 8 minutes. Beat in vanilla. Use immediately.