Preheat oven to 350°F (175°C).

Grease a 9x13 inch baking dish and line with parchment paper, allowing ends to overhang.

Prepare the crust: In a food processor or by hand, combine flour, confectioners' sugar, cornstarch, and salt.

Add cold butter (cubed or grated) and pulse or rub in until the mixture resembles coarse crumbs.

Press the crust mixture firmly into the prepared pan, creating a 1/4 inch layer on the bottom and 1/2 inch up the sides.

Refrigerate crust for 30 minutes.

Bake the crust at 350°F (175°C) for 20 minutes, or until golden brown.

While the crust is baking, prepare the filling.

In a medium bowl, whisk eggs, sugar, and flour until combined.

Stir in lemon zest, lemon juice, milk, and salt.

Reduce oven temperature to 325°F (160°C).

Pour the lemon filling over the warm crust.

Bake for 20 minutes, or until the filling is set but still slightly jiggly.

Cool the bars completely on a wire rack.

Lift the bars out of the pan using the parchment paper overhang.

Slice into bars using a pizza cutter or knife.

Dust with confectioners' sugar before serving, if desired.

Store leftover bars in the refrigerator for up to 2 days.