Combine sugar, water, and salt in a medium saucepan; cook over medium heat, stirring occasionally, until mixture reaches soft ball stage (240°).
Beat egg whites (at room temperature) in a large bowl until foamy. While beating at medium speed of an electric mixer, slowly pour syrup in a thin stream over egg whites. Turn mixer to high speed; beat until stiff peaks form and frosting is thick enough to spread. Frost cake immediately.