Makes: 24 2-inch squares
3 cups sugar
Finely grated zest of 4 lemons
1 ½ cups plus ¼ cup flour
¼ teaspoon kosher salt
12 tablespoons (1 ½ sticks) unsalted butter, cut up
2 teaspoons vanilla extract
1 cup plus 3 tablespoons freshly squeezed lemon juice (start with 8 - 12 lemons)
1. Prep: Lightly butter a 13 x 9 glass baking pan (ideally the sort with a snap on plastic top for storage in fridge). Line with parchment paper, leaving a couple inches overhang. Set oven rack in the lower third of the oven. Heat oven to 325 degrees.
2. Buzz: Measure sugar into the food processor. Drop in zest. Buzz to a fine lemon-scented powder, about 1 minute. Scoop 1/3 cup scented sugar into a small bowl; cover tightly with plastic and set aside. Dump remaining scented sugar into a large mixing bowl; measure 6 tablespoons scented sugar back into food processor. Measure 1 1/2 cups flour and the salt into the food processor. Buzz to combine.
3. Brown: Drop butter into a medium saucepan. Set over medium heat and let melt, foam, then sizzle. When brown specks lurk on the bottom -- after about 15 minutes -- pull pan off heat and cool the pan in an ice bath. Stir in the flour/sugar/salt mixture from the food processor. Stir in the vanilla. Turn this brown-butter pastry into the prepared pan. Pat into the bottom of the pan. To get pastry nice and smooth, cover with a piece of waxed paper, pat again, then remove waxed paper.
4. Bake: Slide pan into oven bake until pastry browns and gives off a nutty fragrance, about 30 to 32 minutes. (Note: all the browning happens at this point so make sure it’s really done. Check the bottom by peering through the glass.) Pull out of oven and cool on a rack at least 15 minutes. This lets pastry set up, so it won’t break up when topped with lemon.
5. Whisk: Meanwhile, whisk remaining 1/4 cup flour into the scented sugar in the mixing bowl. Whisk in juice. Whisk in eggs.
6. Bake: Lower oven temperature to 300 degrees. Return cooled pastry to the oven. Pour egg mixture over pastry (doing this in the oven cuts down on sloshing). Bake until center is just set, about 22 - 24 minutes.
7. Chill: Cool completely. Press a piece of plastic wrap against the lemon top. Cover pan. Chill completely (ideally overnight).
8. Gild: Shortly before serving, uncover. Sprinkle reserved scented sugar over the top. Use an offset spatula to spread evenly. Lift the lemon block out of pan by parchment overhang. Use a long heavy knife to trim edges, leaving an 8 x 12 rectangle. Cut into 24 2-inch squares. For a party, cut these down to 2 x 1 inch bars. Arrange on a platter. Cover loosely and chill. Serve cold.