Preheat oven to 350 degrees F. Lightly butter a 9x13-inch baking pan and line with parchment paper.
Make the Crust: Pulse flour, powdered sugar, cornstarch, and salt in a food processor. Add the butter and process for 8 to 10 seconds and then pulse until the mixture is pale yellow and resembles coarse meal, about three 1-second pulses. Sprinkle the mixture into the lined pan and press firmly into an even layer over the entire pan bottom and ½-inch up the sides. Refrigerate for 30 minutes, then bake until golden brown, about 20 minutes.
Make the Filling: Meanwhile, whisk the eggs, sugar and flour in a medium bowl, then stir in the lemon zest, lemon juice, milk and salt to blend well.
Reduce the oven temperature to 325 degrees F. Stir the filling mixture to reincorporate the ingredients, then pour into the warm crust. Bake until the filling feels firm when touched lightly, about 20 minutes. Transfer the pan to a wire rack; cool to near room temperature, at least 30 minutes. Lift the bars from the pan using the parchment paper, then place on a cutting board to slice into squares. Sift powdered sugar over bars, if desired. Leftover bars can be covered in plastic wrap and refrigerated for up to 2 days.
Serving: Since lemon bars are fine to keep out at room temperature for a number of hours if they are on a serving platter at a party, for example.
Storing: Store cooked lemon bars or leftovers in the refrigerator. Cover tightly with plastic wrap and store for up to 2 days.
Freezing the whole batch: Bake the lemon bars as directed and allow them to cool. Before slicing, you can wrap tightly in plastic wrap, then foil and freeze for up to 1 month.
Freezing individual bars: Once sliced, place lemon bars on a baking sheet and freeze until solid, about 2 hours. Wrap tightly in plastic wrap and place in a freezer-safe bag for up to 2 months.
You can prepare the crust by hand simply by whisking together the flour, powdered sugar, cornstarch, and salt. Then, cut in the butter with a pastry blender (or grate frozen butter into the mixture and rub together quickly with your fingers) until completely incorporated and resembles coarse meal.
The filling needs to be poured into a warm crust straight from the oven, so once the crust is in the refrigerator chilling, prepare the filling so it's ready when the crust comes out of the oven.