Excerpted from Start Here: Instructions for Becoming a Better Cook © 2023 by Sohla El-Waylly with permission of Alfred A. Knopf, a division of Penguin Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.

These lemon squares borrow some magic from Key lime pie. Instead of cooking a traditional curd on the stovetop, here sweetened condensed milk is thickened with lemon juice and yolks. With the power of a posset and baked custard combined, this simple filling is supple and delicate. There’s plenty of lemon juice and zest (I really want to make your mouth pucker), so the nutty brown butter crust keeps things grounded. If you’ve saved a vanilla pod after scraping the seeds for meringue or steeping in cream, add it to the butter while it browns to extract even more of its floral aroma.

Makes 16 squares

Active: 20 mins | Total: about 5 hrs.

Easy | Vegetarian

Special Equipment:

• 8-inch square metal baking pan
• parchment paper

Method:

1. Make the crust: In a medium saucepan, combine the butter and spent vanilla pod (if using) and heat over medium heat until foamy, about 3 minutes. Continue cooking the butter, stirring and scraping frequently with a stiff silicone spatula, until the sputtering has subsided, and the butter solids look deeply browned, 3 to 5 minutes.

2. Remove from the heat, scrape the butter and any browned bits at the bottom into a large bowl. Set aside until cool, slightly solid, but still creamy, about 20 minutes in the refrigerator or 2 hours at room temperature.

3. Heat the oven to 325°F (163°C). Grease an 8-inch square metal brownie pan and line the bottom and two sides with one long sheet of parchment, leaving 1 inch of overhang.

4. Pluck out the vanilla pod (if you used it), scraping off any butter still clinging to it. (Rinse and reserve the pod for another use.) Using a stiff silicone spatula, stir the powdered sugar and salt into the butter. Add the egg yolk and stir until evenly combined. Add the flour and stir until the mixture comes together into crumbs. Use your hands to press the crumbs together into a dough.

5. Dump the dough into the prepared pan. Use your hands and an offset spatula to spread the dough into an even layer. Prick all over with a fork and bake until dry and lightly golden, about 30 minutes. Cool fully, at least 30 minutes. (Tightly wrapped, the crust will keep at room temperature for 5 days.)

6. Increase the oven temperature to 350°F (177°C).

7. Make the filling: In a large bowl, whisk the egg yolks, condensed milk, lemon zest, citric acid (if using), and salt until well combined. Add the lemon juice and whisk until evenly incorporated. Pour over the cooled crust.

8. Bake until the center is just set, 14 to 16 minutes. Transfer to a rack to cool for 1 hour, then place in the fridge to chill fully, at least 2 hours and up to overnight.

9. Run an offset spatula or butter knife along the edges of the pan to loosen the bars. Use the parchment to lift them onto a cutting board. Using a sharp knife and wiping it clean in between slices, cut into 16 squares. Dust with powdered sugar and serve. (Tightly wrapped, leftovers can be stored in the fridge for 5 days.)