In double boiler: 1 cup sugar, 5-6 tablespoons cornstarch, 1/8 teaspoon salt, Add gradually 2 cups water. Stir and cook for 8-10 minutes or until thickens. Cover and cook 10 minutes more stirring occasionally.
Remove from heat. Pour a little over 3 beaten egg yolks. Beat this and return to mixture in double boiler. Cook and stir gently for 5 minutes more. Remove from heat. Beat in: 3 tablespoons butter, 1/3 cup lemon juice, 2 teaspoons grated lemon rind.
Cool the custard by stirring very, very gently to release steam which might condense and thin the filling. Pour it cool into the cold pie shell. Cover with meringue: egg whites whipped until foamy, then add 3 tablespoons sugar and 1 teaspoon cream of tartar. Whip until peaks form. (use the flat side of a spatula and lift to make decorative peaks that brown nicely.) and bake at 350 degrees until peaks are golden brown about 10 minutes.