Place syrup in a small saucepan; cook over medium heat, stirring frequently, until syrup reaches soft ball stage (240°).
Beat egg whites (at room temperature) until foamy. While beating at low speed of electric mixer, add sugar. Turn mixer to medium speed, and continue beating while slowly pouring hot syrup in a thin stream over egg whites. Continue beating until stiff peaks form and frosting is thick enough to spread. Add vanilla; beat until blended. Spread immediately on cooled cake. Sprinkle with grated coconut, if desired.