Preheat oven to 350 degrees F.

Grease an 8-inch square pan with butter, line with parchment paper, and then grease the parchment. Set off to the side for later.

Add the flour, sugar, and salt to a large mixing bowl, whisk to combine. Using a pastry blender, work the chunks of cold butter into the flour mixture until the mixture is powdery but firm if pinched. Or use a food processor to blend until the dough will hold its shape if pinched.

Add the crumbs to the prepared pan. Press the dough into the pan until it is one even layer. Bake the crust for 10 minutes. Remove pan, using a fork, poke holes all along the crust, and then return to the oven. Bake for another ten minutes until barely golden. When done, remove from oven and allow to slightly cool, keeping the oven on.

While the crust bakes, make the filling. Zest the entire lemon. Cut the tops and bottoms off of the lemon so that there are two flat ends. Place the lemon cut side down and then cut away the remaining pith (white part) of the lemon. Cut the lemon into slices, remove any seeds, and then add it to a blender or food processor. Pulse until thoroughly pureed.

Add the butter and puree until smooth and combined, scraping down the sides as needed. Add eggs, cornstarch, and salt. Pulse in short bursts until evenly combined.

Pour the lemon mixture on to the baked and slightly cooled crust. Bake for 35 to 40 minutes until barely light brown on top.

Cool completely in the pan. Cut into 16 squares and then dust lightly with powdered sugar if you want a nice finishing touch.