First Step: Prepare Oven and Pan. Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving overhang to lift bars easily after baking. Lightly grease if desired to prevent sticking.

Second Step: Make the Crust. In a medium bowl, whisk together 2 1/2 cups all-purpose flour, 2/3 cup powdered sugar, and 1/2 teaspoon salt. Pour in melted butter (and vanilla extract if using) and stir until mixture forms a crumbly dough. Press evenly into the bottom of your prepared pan, creating a flat and compact crust layer. Poke holes with a fork optionally to help filling adhere.

Third Step: Bake the Crust. Bake the crust in your preheated oven for about 20 minutes or until edges are lightly golden brown. Keep an eye on the color to prevent overbaking. Remove from the oven and set aside briefly while preparing the filling.

Fourth Step: Prepare the Lemon Filling. While the crust bakes, place granulated sugar and lemon zest in a food processor, pulsing until pale yellow and fragrant. If no processor is available, finely mince zest and mix thoroughly with sugar by hand. In a large bowl, whisk together lemon sugar mixture, remaining 1/2 cup flour, fresh lemon juice, and eggs until smooth and fully incorporated.

Fifth Step: Combine and Bake. Pour the lemon filling evenly over the warm crust. Return the pan to the oven and bake for an additional 25 minutes. Rotate the pan halfway through to promote even baking. The center should be mostly set but may have a slight jiggle.

Final Step: Cool, Chill, and Serve. Remove the bars from the oven and allow them to cool completely at room temperature for about one hour. Then, chill them in the refrigerator for at least 2 hours, or longer, so they firm up nicely. Just before serving, dust generously with powdered sugar. Using a sharp knife dampened with water, cut into 15 bars, wiping the blade between cuts for clean slices.