In the top of a double boiler stir together sugar and 1/3 cup water. Add egg whites and cream of tartar. Beat with an electric mixer on low speed 30 seconds. Place over boiling water (upper pan should not touch water). Cook, beating constantly on high speed, about 7 minutes or until stiff peaks form. Remove from heat; add vanilla. Beat 2 to 3 minutes more or until easy to spread. Spread on cupcakes. Top with coconut. Serve the same day.