1-First Step: Prep and preheat Preheat your oven to 350°F. Line a 9×13-inch baking dish with parchment paper, leaving an overhang on two sides for easy removal. Measure out ingredients so everything is ready this mise en place helps you move smoothly from crust to filling.

2-Second Step: Make the shortbread crust In a bowl, whisk together 2 1/2 cups of the all-purpose flour, 2/3 cup powdered sugar, and 1/2 teaspoon salt. Add the 12 tablespoons melted unsalted butter and, if you like, 1 teaspoon vanilla for a subtle depth of flavor. Mix with a spatula until the dough comes together and looks evenly moistened. Press the dough evenly into the prepared pan using your fingers or the bottom of a measuring cup to compact it. Bake the crust for about 20 minutes, or until it is light golden at the edges. The oven time can vary by a few minutes depending on your oven, so watch for a consistent light color.

3-Third Step: Prepare the lemon-sugar mixture While the crust bakes, process 3 cups of sugar with 3 tablespoons lemon zest in a food processor until the zest is incorporated and the sugar looks pale yellow. If you do not have a processor, finely mince the zest and mix well into the sugar. This step releases the oils and gives a brighter lemon flavor to the filling.

4-Fourth Step: Mix the filling In a large bowl, whisk together the lemon-zest sugar, the remaining 1/2 cup all-purpose flour, 1 cup fresh lemon juice (240 mL), and 8 large eggs at room temperature. Whisk thoroughly until smooth and slightly frothy; this helps a silky texture.

5-Fifth Step: Combine and bake Pour the lemon filling over the warm crust. Place the lined pan on a baking sheet to catch any drips and for even heat distribution. Bake at 350°F for about 25 minutes, rotating the pan halfway through baking for even color. The filling should be set but with a faint, custard-like jiggle in the center. If the filling looks too runny, bake an extra 3 to 5 minutes.

6-Final Step: Cool, chill, and cut Remove the pan from the oven and cool at room temperature for 1 hour. Then chill in the refrigerator for 2 hours to let the filling fully set. Dust the top with powdered sugar if desired. Use a sharp, hot, and damp knife to cut clean squares, wiping the blade between cuts. For best texture, chill until firm before slicing.