Mix cornstarch with ½ c cold water in top of double boiler; blend in sugar and salt. Add remaining water. When all is well-blended, stir constantly over low heat mixture until the mixture boils.Cover and cook over boiling water 10 minutes.
Gradually pour hot mixture over beaten egg yolks, stirring constantly.
Return to double boiler and cook 2 minutes longer.
Remove from heat; add butter, lemon juice, and lemon rind. Mix well; cool, and pour into pie shell.
Beat 3 egg whites until stiff. Gradually add 6 T sugar. Pile on top of pie.
Bake 15 minutes in a 325º oven.