Combine sugar, water, and salt in a medium saucepan. Cook over medium heat, stirring frequently, until mixture comes to a boil and sugar is dissolved. Continue cooking, stirring frequently, until the mixture reaches soft ball stage (240°).
Beat egg whites (at room temperature) until foamy. While beating at medium speed of electric mixer, slowly pour hot syrup in a thin stream over egg whites. Turn mixer to high speed; continue beating until stiff peaks form and frosting is thick enough to spread. Add vanilla; beat until blended.