Place sugar, salt, cream of tartar and boiling water in top of double boiler. Place over boiling water. Add unbeaten egg white and commence immediately to beat the mixture with a mixer. Beat until thick and smooth. Remove from heat and add flavouring. To test the frosting cut through with a silver knife. The frosting should retain the clean cut. If it runs together it has not been cooked long enough. Cook it until the test works.
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