Combine sugar, egg whites (at room temperature), syrup, and cream of tartar in top of a large double boiler; add water, and beat on low speed of electric mixer for 30 seconds or just until blended.
Place over rapidly boiling water; beat constantly on high speed of electric mixer about 7 minutes or until stiff peaks form. Remove from heat. Add vanilla; beat 2 to 3 minutes or until frosting is thick enough to spread. Spread on cooled cake.