Place an oven rack in the middle position, preheat your oven to 350 F (175 C), and line a 9" x 13" baking or Pyrex dish with parchment paper.

Prep your lemon zest and fresh lemon juice. You'll need the zest from 1 lemon, which is about 1/2 tbsp of finely grated lemon zest. And juice from 3 lemons, which is about 1/2 cup of freshly squeezed lemon juice. Be sure to strain the lemon juice after squeezing to ensure there are no seeds.

Take a food processor and add in 3 cups of granulated sugar and the 1/2 tbsp lemon zest prepared earlier. Pulse until the zest and sugar are fully incorporated.

Transfer this lemon-sugar mixture into the bowl of your stand mixer. Add in 8 room temperature eggs, and using the whisk attachment, whisk the sugar and eggs together on high speed till the mixture looks pale yellow, very fluffy, and doubled in volume. This will take about 2 to 3 minutes on high speed. This process dissolves the sugar and incorporates air, which will form a flakey crust on top when baked.

When the whipped eggs and sugar are foamy, thick, and pale, stop the mixer, and detach the bowl from the stand mixer. Add in the 1/2 cup of freshly squeezed lemon juice prepared earlier, and with a sieve, sift in 1/2 cup of all-purpose flour and 1 tsp of baking powder. Use a spatula to gently fold in the dry ingredients and lemon juice till everything is fully incorporated. Be sure to scrape the sides and bottom of the bowl to ensure everything is mixed in. Set this mixture aside and let it rest at room temperature while you prepare the crust.

Take a large mixing bowl and sieve, add in 3 cups of all-purpose flour, 3/4 cup of powdered sugar, and 1 tsp of salt. Sift this together, then whisk the dry ingredients together to combine. Add in 14 tbsp of melted unsalted butter and 2 tsp of vanilla extract. Mix this together with a spoon till everything is well combined.

This shortbread dough should look like crumbles. By hand, sprinkle these crumbles evenly into the prepared baking dish and spread the crumbles into an even layer with your fingers, and using the bottom of a measuring cup, press it down firmly into a flat layer, doing your best to make it as even as possible and getting rid of any gaps. It should make a very tight and compact layer with no gaps. Use an offset spatula to press down the dough at the corners and edges of the pan. Bake in your preheated oven at 350 F (175 C) for 20 minutes, or until the crust is a light golden brown.

When the crust is done, take it out of the oven. Working quickly so the crust doesn't cool off too much, use a fork to make small indents all over the top surface of the warm crust. But the indentations should not go all the way through the crust - only about halfway through the crust. Immediately pour the lemon filling mixture into the still warm crust and return the pan to the oven. Bake at 350 F (175 C) for 30 to 40 minutes. It's done when the top is golden brown and the center is no longer jiggly.

Let it cool in the pan on a metal rack till it comes completely to room temperature.

Once it reaches room temperature, chill it in the fridge for a minimum of 4 hours, preferably overnight. After it's been chilled, use the parchment paper as handles to lift the entire block out of the baking dish and onto a chopping board and peel back the parchment paper sides. Dust with additional powdered sugar. We like to slice it into 16 bars, cutting the 13" side equally into 4, and the 9" side equally into 4. Enjoy!