In a large saucepan, bring 1 inch of water to a simmer. In a large stainless-steel bowl, combine the sugar, egg whites, 1/4 cup of water, the cream of tartar and the salt. Set the bowl over the simmering water and beat the egg whites with a whisk or handheld mixer until the frosting holds stiff peaks, about 5 minutes.
Remove the bowl and continue to beat until cooled and billowy, about 2 minutes more. Beat in the vanilla and frost the cake.