Preheat the oven to 325F and line a 9-x-9-inch baking pan with enough foil or parchment paper to hang over two of the sides (these will be used as handles later, to lift the bars out of the pan).

Make the shortbread base: whisk together the flour, granulated sugar, powdered sugar, lemon zest and sea salt in a large bowl. Using two knives, cut the butter cubes into the flour mixture until a crumbly dough forms. Press the dough into the prepared pan and bake until the shortbread is pale golden all over, 30-35 minutes.

While the shortbread is baking, prepare the lemon curd: Grate ½ tablespoon zest from lemons and set aside. Squeeze the lemons to yield ¾ cup juice.

In a small saucepan, whisk together the lemon juice, sugar, eggs and yolks, cornstarch and fine sea salt over medium heat until the mixture has boiled and thickened, 2-5 minutes. (Make sure the mixture comes to a boil or the cornstarch won’t activate. But once it boils do not cook for longer than 1 minute, or you risk the curd thinning out again.) Remove from heat and whisk in the butter, olive oil and lemon zest.

When the shortbread is ready, take it out of the oven and carefully pour the lemon curd onto the shortbread base in an even layer. Return the pan to the oven. Bake until topping is just set, 10-15 minutes more. Allow the bars to cool to room temperature, then refrigerate until cold before cutting into bars. Sprinkle with powdered sugar and flaky sea salt right before serving.