Combine 1/4 cup sugar and 1/4 cup water in a medium saucepan over medium heat; slowly bring mixture to a boil, making sure sugar dissolves. Cook 8 to 10 minutes or until syrup is a dark amber color. Stir in corn syrup; bring to a boil. Cook, without stirring, until mixture reaches soft ball stage or candy thermometer registers 240°.
Meanwhile, beat egg whites, salt, and remaining 1/2 cup sugar until soft peaks form. Continue to beat, slowly pouring hot syrup in a heavy stream down the side of the bowl, until frosting is thick enough to spread.