In a heatproof mixing bowl (or stand mixer bowl), combine egg whites, sugar, water, cream of tartar, and salt. Whisk to combine.

Set the bowl over a saucepan of simmering water (top of a double boiler) and whisk constantly until the mixture reaches 140°F. The sugar should be dissolved and the mixture foamy.

Immediately move the bowl to a stand mixer and whip with the whisk attachment on high speed for 7 minutes, or until the frosting has a fluffy texture, stiff peaks, and the bowl is no longer warm.

Add the vanilla extract and beat for another 30 seconds to finish.