Preheat the oven to 350°F. Line a 9x13 inch baking pan with parchment, leaving a little overhang on the sides for easy removal. It makes removal and cleaning up so much easier.
Make the crust:
In a large bowl, whisk together the flour, sugar, and salt. Add in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. (You can cut the butter into small cubes to help add into the flour mixture).
Press the dough evenly into the prepared pan.
Bake for 20-22 minutes, or until the edges are just starting to turn golden. Remove from the oven but leave the oven on.
Mix the filling:
While the crust bakes, whisk together the sugar, flour, and cornstarch in a medium bowl.
Add the eggs, lemon zest, lemon juice, and salt. Whisk gently until fully combined but not frothy, this keeps the top smooth and prevents bubbles.
Pour the filling over the hot crust and return the pan to the oven. Bake for 30-35 minutes, or until the center is just set and no longer jiggles when gently shaken.
Cool and Chill:
Remove from oven, and let the bars cool completely at room temperature, then refrigerate for at least 1-2 hours before slicing. This ensures clean edges and a firm filling. Wipe the knife between cuts for the cleanest edges.
Dust the top lightly with powdered sugar after cutting into squares or bars.