PLACE a rack in middle of the oven and preheat your oven to 350 degrees. Cut two 12-inch lengths of parchment paper, and trim each to fit the bottom of an 8-inch square baking pan. Press the first sheet into the bottom and up the sides of the pan in one direction, then use the second sheet to line the rest of the pan, running it perpendicular to the first sheet. Lightly butter exposed parts of parchment or coat them with a nonstick cooking spray. OR spray or butter just the pan and don't use parchment paper. Set the pan aside.

BLEND flour, 1/3 cup sugar and 1/4 teaspoon salt together in the work bowl of the food processor. Add 1 stick butter and pulse until the mixture is powdery, but if firmly pinched, will hold the pinched shape. Turn the dough crumbs into the prepared baking pan and press dough evenly across the bottom and about 1/2 inch up the sides. Prick the dough all over with a fork and bake for 20 minutes, or until lightly browned. Should any parts bubble up, gently prick them again with a fork. Leave the oven on. Let cool slightly before pouring lemon mixture over it.

CUT lemon in half. If the white part of the skin is thicker than 1/4 inch, then remove the skin from one half of the lemon. If not, you are good to go. (The bitterness of a thick pith will overwhelm the bars.) Cut the lemon halves into thin rings and discard any seeds. Toss the lemon rounds – lemon flesh and peel – in the bowl of the food processor, add remaining 1 1/3 cups sugar, and run the machine until the lemon is thoroughly pureed, about 2 minutes. Add the remaining stick of butter and again run the machine until the mixture is smooth, scraping down the sides of the work bowl as needed. Add eggs, cornstarch and remaining 1/4 teaspoon salt and pulse the machine in short bursts until the mixture is easily combined.

POUR lemon mixture over the crust and bake for 15 to 25 minutes, or until the filling is set, puffed and golden. Let pan cool completely on rack or in the fridge. Gently cut around the outside of the parchment paper to make sure no sides have stuck, then gently use the parchment “sling” to transfer the bars from pan to cutting board.