Butter the bottom and sides of a 9 by the 13-inch metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up two sides of the pan and overhang slightly on both ends, and then lay second sheet crosswise over it.. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.

In the bowl of a food processor fitted with steel blade. Pulse flour, confectioners’ sugar, cornstarch, and salt. Add butter and process 8 to 10 seconds, until mixture is pale yellow and resembles coarse meal. Process again for three 1-second bursts. Sprinkle mixture into lined pan and press firmly with fingers into even layer. Refrigerate for 30 minutes. Preheat oven to 350F. Bake until golden brown, about 20 minutes.

While the crust is baking, make the filling: In a medium bowl whisk together the eggs, sugar, and flour.  Stir in lemon juice, zest, milk, and salt; set aside.

Reduce oven temperature to 325 degrees. Stir filling mixture and pour into warm crust. Bake until filling feels firm when touched lightly, about 20 minutes. Transfer pan to wire rack to cool completely. Cut into bars and sprinkle with confectioners’ sugar, if desired.