Combine sugar, egg whites (at room temperature), syrup, and cream of tartar in the 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 boiling water; beat constantly on high speed about 7 minutes or until stiff peaks form. Remove from heat, and add marshmallows. Continue beating until frosting is thick enough to spread.
Oxmoor House Homestyle Recipes
