• Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper (with enough paper so you can use it to lift the bars from the pan after they are baked) and spray it with cooking spray.

Make the Crust

• Place the flour, sugar, and salt in a food processor. Add the butter and pulse until is worked into the ingredients and the flour looks powdery. You should be able to pinch some of the dough together and it should hold its shape. Transfer the flour and butter mixture to the baking pan and press it into an even layer in the bottom of the pan and approximately one inch up the sides. Using the tines of a fork, poke the dough all over to make little holes. Bake the crust for 20 minutes, or until it's lightly browned.

Make the Filling

• Cut the lemon into thin slices. Remove the seeds from the lemon and discard them. Place the lemon slices in the food processor, add the sugar, and puree the lemon. This will take a couple of minutes and you may need to stop the food processor a few times to scrape down the bowl. Process the lemon and sugar until you have a smooth puree. Add the butter, eggs, cornstarch, and salt and pulse the mixture a few times until combined.
• Pour the lemon mixture over the crust, transfer the pan to the oven, and bake it for 35 to 40 minutes, or until the filling is set and the bars turn a very light brown on top. Cool the bars completely in the pan before lifting them out (use the parchment paper to transfer them). Slice into 16 squares.