Preheat oven to 325F (160C) and line a 9x9” square baking pan with enough parchment paper (do not use aluminum foil) so that some is hanging over the sides (you want to be able to lift the bars out of the pan once they’ve cooled completely).
To prepare your cookie crust, combine flour, sugar, cornstarch, and salt in the basin of a food processor¹ and pulse until well combined.
Scatter butter pieces evenly over the dry ingredients and pulse until the mixture just begins to clump together (about 10 1-second pulses). In a small dish, whisk together egg yolk and vanilla and pour over dry ingredients. Pulse again until mixture begins to cling together. It will still be crumbly, but if you press the dough together with your thumb and forefinger it should hold together (about 18 1-second pulses).
Pour dough/crumbles into the bottom of your prepared pan and use your hands or the back of a spoon to press evenly into the bottom of the pan to form an even crust (I like to place a piece of wax paper over the dough and then smooth it with a spoon for an even layer, see my video for a visual of how I do this).
Transfer to 325F (160C) oven and bake for 25 minutes or edges are beginning to turn light golden brown. While crust is baking, prepare your lemon topping.
While the crust is baking, combine eggs, egg yolks, lemon juice, sugar, and salt in a medium-sized saucepan. Add butter and transfer to the stovetop over medium/low heat. Stir constantly (do not let the curd boil) until thickened and mixture coats the back of a spoon.
Remove from heat and immediately pour through a fine mesh strainer into a heatproof bowl.
Once crust has finished baking, use a fork to poke holes over the crust, only piercing about halfway into the crust. Pour curd evenly over the surface of the crust and return to 325F (160C) oven and bake for another 10-15 minutes or until topping is set and only slightly jiggly in the center.
Allow to cool to room temperature, then transfer to fridge to cool completely, about 2 hours. Use the overhanging parchment to lift the bars out of the pan and then dust evenly with powdered sugar and cut into squares before serving.
¹If you don’t have a food processor you can use a pastry cutter instead. Combine all dry ingredients and then cut butter into the mixture until it resembles coarse crumbs. Whisk together egg yolk and vanilla and then stir well into the batter. It will take a bit more work to do by hand, but mixture will come together with some effort.