Combine sugar, corn syrup, and 1/2 cup water in a heavy 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. Slowly pour hot syrup in a thin stream over egg whites while beating at medium speed of electric mixer. Turn mixer to high speed, and continue beating until stiff peaks form and frosting is thick enough to spread. Add vanilla; beat until blended. Spread immediately on cooled cake.