1. Heat oven to 160 degrees C and line a 23cm square baking pan with enough baking paper to hang over two of the sides (to be used as handles later to lift the bars out of the pan).
2. To make the shortbread base, pulse together the flour, granulated sugar, icing sugar, lemon zest and salt in a food processor, or whisk together in a large bowl. Add the butter and pulse (or use two knives or your fingers) to cut the butter into the flour until a crumbly dough forms. Press the dough into the prepared pan and bake until the shortbread is pale golden, 30 to 35 minutes.
3. While the shortbread is baking, prepare the lemon curd: Grate 1/2 tablespoons of zest from the lemons and set it aside. Squeeze the lemons and set aside 3/4 cup of juice.
4. In a small pot, whisk together the lemon juice, sugar, eggs and yolks, cornstarch and fine sea salt over medium heat until boiling and thickened, 2 to 5 minutes. Ensure the mixture comes to a boil and stays there for 30 seconds or so; it should thicken before removing from the heat.  Once the curd boils do not cook for longer than 1 minute or you risk the curd thinning out again. Remove the curd from the heat and strain it into a bowl. Whisk in the butter, olive oil and lemon zest.
5. When the shortbread is ready, remove it from the oven and carefully pour the lemon curd onto the shortbread base; then return the pan to the oven. Bake until the curd is just set, 10 to 15 minutes more. Allow it to cool to room temperature, then refrigerate until cold before cutting into bars. Sprinkle with icing sugar and flaky sea salt right before serving.

Enjoy!