In a medium saucepan, combine cornstarch, sugar, water and salt and whisk well to combine. Turn heat on to medium and stir constantly. Bring mixture up to a boil, then boil for 1 full minute. Remove from heat and gradually add 1 heaping spoon full of the hot mixture to the egg yolks, whisking the egg yolk mixture constantly. Return egg mixture in saucepan and turn heat down to low and cook for 1 full minute more. Remove from heat and gently stir in butter, lemon juice and zest until well combined. Pour mixture into pie shell(s) and top with meringue while filling is still hot. Make sure meringue goes right up to the edge of the crust. Bake for 10-12 minutes or until meringue is golden. Remove from oven and cool on a wire rack. Cool completely before slicing.
