Preheat the oven to 350°F Line the bottom and sides of a 9x13 glass baking dish with parchment paper, leaving an overhang on the sides to lift the finished bars out. Set aside. Make the shortbread crust. Mix softened butter, sugar, vanilla extract, and salt together in a medium bowl. Add the flour and stir to completely combine. The dough will be thick and a bit crumbly.
Press it down firmly into prepared pan, making sure the layer of crust is nice and even. Bake for 22-25 minutes or until the edges are slightly browned. Remove from the oven. Gently poke some holes in the crust with a fork.
Make the lemon curd. Using a mesh stainer, sift the flour and the sugar to help prevent clumping. Whisk in the eggs, then the lemon juice until completely combined.
Pour the lemon curd over the warm crust. Bake the bars for 22-24 minutes or until the center is relatively set and no longer jiggles. (Give the pan a little shake with an oven mitt to test.) It's okay if they are still slightly jiggly, they will set in the refrigerator more.
Remove bars from the oven and cool completely at room temperature. I usually cool them for about 1 hours at room temperature, then stick in the refrigerator for 1-2 more hours until pretty chilled. I recommend serving chilled.
Dust with confectioners’ sugar and cut into squares before serving. For neat squares, wipe the knife clean between each cut. Enjoy!