• Preheat the oven to 350 °F (175 °C). Butter and line an 8x8-inch (20x20 cm) with parchment paper, making sure that you leave overhangs on all sides. You can use metal binder clips to secure the parchment overhangs to the edge so that they stay upright and in place. Set aside.

Shortbread cookie crust

• In a food processor, combine the flour, sugar, and salt. Let it run so they are well mixed.
• Drop in the cubes of butter, and then pulse to form a coarse sandy mixture.
• Dump the mixture in the prepared pan and press it well into the corners and edges. You can use a flat-bottomed glass to even out the surface and to help you press down the crust firmly.
• Bake the crust on the middle rack for 20 to 30 minutes. The edges should be golden brown. If you like a blonder shortbread crust, bake it for 20 minutes, for a more caramelized cookie crust, bake it for 30 minutes.

Lemon curd filling

• While the crust is baking, cook the lemon curd filling.
• Prepare a fine-mesh sieve and place it over a 4-cup liquid measuring cup with a pouring spout or a bowl with a pouring spout. Set aside.
• In a medium saucepan, combine the lemon juice with half the sugar. Heat it on medium–high.
• In a large bowl, whisk together the egg yolks, whole eggs, and the rest of the sugar until light in colour.
• When the lemon juice begins to steam and is almost simmering, turn the heat off, and pour it over the whisked eggs and sugar, while whisking. Combine everything together and when the filling is well mixed, transfer it back to the saucepan.
• Turn the stove back on to medium heat and whisking constantly, heat the filling until it's 77–82 °C (170–180 °F). The lemon curd should be thick, creamy and coat the back of the spoon.
• Immediately take the curd off the stove and incorporate the cubes of butter while still whisking.
• Once the butter has all been incorporated, pour the curd through the strainer into the measuring cup.
• When the crust has baked sufficiently, take it out of the oven and place the pan on a cooling rack.
• Pour the curd over the hot crust and smooth the surface with an offset spatula if needed. Make sure the curd covers the entire surface, from one edge of the pan to the other.
• Place the pan back in the oven and bake for 10 minutes more. The edges of the curd will appear set but the centre will still be wobbly. Note that this baking step is optional but it can help the edges of the bars set more firmly, making it easier to separate them from the parchment paper when cutting.
• Set the pan on a wire rack to let it cool completely for a few hours. Once cooled to room temperature, chill in the fridge overnight. You may want to leave the pan unwrapped to avoid any condensation, but if any water forms on the surface of the lemon curd, you can easily blot it with a paper towel.
• Once chilled overnight, you can lift the bars out of the pan using the parchment overhangs. Carefully peel off the parchment from the edges of the bars, and place on a cutting board to cut into 16 equal bars. Decorate with powdered sugar before serving.