Combine sugar, egg whites (at room temperature), cream of tartar, and salt in top of a large double boiler; add water, and beat on low speed of an electric mixer for 30 seconds or just until blended.
Place over boiling water; beat constantly on high speed of electric mixer about 7 minutes or until stiff peaks form. Remove from heat. Add vanilla; beat until frosting is thick enough to spread.
Oxmoor House Homestyle Recipes