In the top of a double boiler, whisk together sugar, egg whites, salt, and cream of tartar. Cook over simmering water, stirring occasionally, until a candy thermometer registers 140°F (60°C).

Transfer mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat at high speed until mixture has cooled and tripled in volume, 5 to 7 minutes. Beat in vanilla.

Reserve 1 cup meringue (about 95 grams) to make Meringue Oyster Mushrooms, if desired. Use remaining meringue immediately.