Adjust an oven rack to the middle position and preheat the oven to 350.

Cover a 9x13-in metal baking pan with heavy-duty aluminum foil. Push the foil neatly into the corners and up the sides of the pan, ensuring overlap for easy removal. Spray with nonstick cooking spray.

Place the flour, cornstarch, salt, and Confectioners' sugar in a food processor and mix for a few seconds. Add butter and blend until mixture resembles coarse meal. Press mixture into prepared pan, building up a thin edge around the sides. Refrigerate for 30 minutes or freeze for 15 minutes.

Bake the crust until lightly golden, 15 to 20 minutes.

In a large bowl, whisk together the eggs, sugar, lemon juice, lemon zest, and flour. Ensure no lumps of flour remain.

Pour lemon mixture over the crust and bake another 30 to 35 minutes, or until the topping is set and firm. Cool on a rack to room temperature.

To cut, use the foil overhang to lift the baked square out of pan. Carefully loosen foil from edges, cut into squares or triangles. Dust with Confectioners' sugar.

Store bars in a covered container in the refrigerator for up to 4 days. Serve chilled or at room temperature.

Freezer-Friendly: Freeze bars for up to 3 months. After cooling, cover tightly and freeze.