Place egg whites, sugar, cream of tartar and water in top of 2-quart double boiler. Set over rapidly boiling water and beat constantly at full speed until frosting will stand up in stiff peaks, about 6 minutes with rotary beater and about 4 minutes with electric beater. Scrape from side and bottom of double boiler occasionally. Remove from boiling water. Add flavoring and continue beating until thick enough to spread.