For the crust:
8 tablespoons (1 stick) unsalted butter, softened and cut into chunks
1/2 cup powdered sugar
1 cup all-purpose flour
3/4 cup almond meal (or more flour)
1/2 teaspoon salt
For the lemon curd:
4 large eggs
4 large egg yolks
1 cup granulated sugar
Zest from 5 to 6 lemons
3/4 cup fresh-squeezed lemon juice (from 5 to 6 lemons)
1/4 teaspoon salt
8 tablespoons (1 stick) unsalted butter, softened and cut into cubes
Powdered sugar, for dusting
Heat the oven to 350°F with a rack in the middle position. Line an 8x8-inch baking dish with parchment, letting the excess parchment hang over the sides of the pan.
First, prepare the shortbread crust. Beat together the butter and powdered sugar in a stand mixer (or with a hand mixer) on medium-high speed until they are completely combined, turn pale yellow, and resemble creamy frosting. Stop the mixer and scrape down the sides of the bowl as needed while mixing.
Add the all-purpose flour, almond flour, and salt to the bowl. Beat on low speed just until all the flour is incorporated and the mixture comes together into a dough. The dough will seem fairly crumbly, but should hold together when squished in your hand.
Press the dough into the baking dish. Use the bottom of a cup to make sure the layer is as compacted as possible and in an even layer. Prick with a fork all over. Bake for 20 to 25 minutes, until the edges of the shortbread turn golden-brown.
While the shortbread crust is baking, prepare the lemon curd. Whisk together the eggs, yolks, and sugar in a small (2-quart) saucepan, then whisk in the lemon zest, lemon juice, and salt to form a smooth, liquidy mixture. Set a small strainer over a mixing bowl and place next to the stove.
Place the pan with the lemon mixture over medium heat. Stir, gently but continuously, until the mixture thickens enough to coat the back of a spoon and registers about 155°F on an instant-read thermometer; this should take 8 to 10 minutes. Be sure to scrape the bottom and edges of the pan as you stir. If you notice the mixture starting to coagulate and clump up, immediately remove the pan from heat.
Strain the lemon curd into the bowl, removing the zest and any clumps from the curd. While the curd is still warm, stir in the butter. Stir until the butter is completely melted.
When the shortbread crust is ready, remove it from the oven and pour the curd over top. (It's okay if one is finished before the other, though ideally, they will finish at close to the same time.) Put the pan back in the oven and bake at 350°F until the edges of the curd are set, but the middle is still jiggly, 10 to 15 minutes