In a 1-quart saucepan, blend cornstarch and cold water. Stir in boiling water. Cook, stirring constantly, over high heat until clear and thickened like pudding. Cool completely.
Beat egg whites till foamy. Add salt. Gradually add sugar, beating on high speed until stiff. Lower speed, add vanilla and gradually beat in the cold cornstarch mixture. Beat at high speed till thick and light. Spread on a 9-inch pie.
Bake at 425 degrees for 5 minutes, or till peaks are golden brown. Cool completely, then refrigerate to chill completely, 2 to 4 hours. Makes 8 servings.