Fill a saucepan with 2 inches of water and bring to a boil, then reduce heat to low to maintain a steady simmer.
Place the sugar, water, egg whites, cream of tartar, and salt in a large, heatproof mixing bowl and mix with a hand mixer on medium-low speed for 30 seconds to combine.
Place the mixing bowl on top of the saucepan of simmering water (the bottom of the bowl should not touch the water) and beat on high speed for 7 minutes, or until stiff and glossy.
Remove the bowl from the heat, add the vanilla, and beat for an additional 2 minutes. Use the frosting immediately, as it will start to set and be difficult to spread. Cakes or cupcakes frosted with this should be served within 12 hours of frosting, or stored in the refrigerator, covered, for up to 2 days.