In the heatproof bowl of a stand mixer, whisk together sugar, egg whites, cream of tartar, and salt. Place bowl over a saucepan of simmering water, and cook, whisking frequently, until sugar completely dissolves and an instant-read thermometer registers 120°F (49°C) to 130°F (54°C).

Carefully return bowl to stand mixer. Using the whisk attachment, beat at high speed until stiff peaks form, 2 to 3 minutes. Beat in vanilla. Use immediately.