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.
Notes
Clean equipment is key: Any trace of grease or oil in your bowl will stop the egg whites from whipping properly. Use glass or stainless steel for best results.
Watch the temperature: Heating the mixture to around 140°F ensures the sugar dissolves and gives you that glossy, stable meringue texture.
Use it right away: This frosting is at its fluffiest when fresh—spread it on cakes or cupcakes as soon as it’s done for the prettiest finish.