Moisten the cornstarch with the cold water. Stir in the boiling water. Cook and stir until thickened and clear - done in seconds. Cool completely. Beat the egg whites until firm, gradually add the sugar, then beat in the cornstarch mixture and vanilla. Presto! Makes enough for one large pie or 2 smaller ones. Spread it to the edge of the filled pie and put it in the oven at 425F for 5 to 6 minutes to gild the top.