Preheat the oven to 350 degrees F. Line an 8x8-inch baking pan with parchment paper, and spray the paper with cooking spray. Set aside.
Combine the flour, sugar, and salt in the bowl of a stand mixer. Pulse a couple of times until things are combined. Add the cold butter and pulse until the mixture is powdery. If you pinch the mixture, it should hold it's shape.
Press the crust into the bottom of the prepared baking dish, allowing the crust to go about 1/2 an inch up the sides of the pan. Poke it all over with a fork and bake for 20 minutes.
Start by cutting the lemon in half. If the white part of the skin is thick (i.e., 1/4 inch or more), peel the skin from half of the lemon before continuing. The second half can be used as it is. If the white pith isn’t too thick, just proceed to the next step without peeling anything.
Cut the lemon halves into thin slices, removing and disposing of any seeds. Put the lemon slices in the bowl of the food processor. Add the sugar and run the food processor until the lemon is thoroughly puréed (about 2 minutes).
Add the butter and run it again until the mixture is smooth. Scrape the sides of the work bowl as needed.
Add the eggs, cornstarch, and salt to the food processor. Pulse until well-combined.
Pour the lemon filling over the crust and bake for 35-40 minutes. The bars are done when the filling jiggles only a little bit in the middle.
Let the pan cool completely on a rack. Once cooled, cut the bars into 16 squares. Dust with powdered sugar.
Store the bars in the fridge if not eating immediately.