In a medium glass bowl, combine the sugar, water, egg whites, cream of tartar, and salt.
Set a saucepan of water on the stove to come to a simmer. Meanwhile, beat egg mixture with an electric mixer on low for 30 seconds. Set bowl over simmering pan of water, forming a double boiler and continue beating on high for 7 minutes, until stiff and glossy.
Remove from heat, add vanilla, and beat 1 to 2 more minutes.
Use immediately, frosting will set quickly.