Combine sugar and water in a medium-size heavy saucepan. Cook over medium heat, stirring frequently, until mixture comes to a boil and sugar is dissolved. Continue cooking, stirring frequently, until mixture reaches soft ball stage (240°).
Beat egg whites (at room temperature) until foamy. While beating at medium speed of electric mixer, slowly pour half of the hot syrup in a thin stream over egg whites. Return remaining syrup to heat, and continue cooking until mixture reaches hard ball stage (260°).
Continue beating at medium speed of electric mixer, and slowly pour remaining syrup in a thin stream over egg white mixture. Turn mixer to high speed, and beat until thick enough to spread. Add orange rind and juice; beat until blended. Spread immediately on cooled cake.