Combine sugar, egg whites (at room temperature), syrup, and cream of tartar in top of a large double boiler; add water, and beat at low speed of an electric mixer 30 seconds or just until blended.
Place over rapidly boiling water; beat constantly at high speed of electric mixer 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.
Oxmoor House Homestyle Recipes