Preheat the oven to 350° F. Line an 8x8 pan with parchment paper (for easy removal later) and set aside.
make the crust
In a microwave safe bowl, heat the butter until melted. Add the sugar and mix until combined. Add the vanilla and mix again. Pour in the salt and flour and mix until everything is combined (the dough will be pretty thick).
Pour the dough into the pan and spread it evenly into the bottom. Bake this for about 20 minutes, until the edges are just slightly starting to brown.
make the lemon curd
While the crust is baking, make the lemon curd by combining the sugar and flour in a large bowl, making sure there are no clumps. Add in the eggs, lemon zest, and lemon juice. Whisk until everything is well combined, and then let it sit for 10 minutes.
assemble and bake
Before pouring the lemon mixture onto the crust, take a fork and gently press holes into the top of the crust (making sure not to poke all the way through). Then use a rubber spatula to press the crust back towards the sides of the pan, sealing the edges that might have pulled away while baking. This is to make sure the lemon curd doesn't sink into the cracks around the edges of the pan.
Give the lemon curd mixture a good mix one more time and then gently pour the lemon curd onto the crust. Place back into the oven the bake for 20-22 minutes until the curd is set (but still jiggles like firm jello). Remove and place on a wire cooling rack.
cool & slice
Allow the bars to cool COMPLETELY in the pan (about an hour), and then place in the fridge for a few hours to chill. Remove from the pan by pulling up the parchment paper and use a sharp knife to cut into squares. Dust with powdered sugar and enjoy!
spoon and level your flour- I always recommend spooning and leveling your flour when baking. Scooping the measuring cup right into the flour will actually compact the flour and give much more than the recipe calls for. To spoon and level, use a large spoon to gently scoop the flour into your measuring cup. Once it's full, use a flat edge (like a butter knife) to gently scrape across the top of the measuring cup.
make sure to seal the crust- As I said above, I learned this one the hard way (aka many recipe tests for these lemon bars). When the shortbread crust bakes, it pulls away from the edges of the pan, leaving cracks for the lemon curd to sink into which gives you upside-down lemon bars. Before pouring the lemon onto the crust, take a rubber spatula and gently press the crust back towards the edges of the pan to seal those cracks.
don't overbake the lemon bars- Overbaking lemon bars can actually cause your lemon curd to curdle and will give you an almost grainy texture. Make sure to watch your lemon bars and take them out of the oven when they pass the "jiggle test"- they should jiggle like firm jello when they are done.
use a sharp knife to cut- when cutting lemon bars, use a sharp knife to get clean cuts. I prefer using a big chef's knife to get all the way through. I also like to clean my knife off with warm water and a towel in between cuts to keep the cuts nice and clean (you don't have to do this but it helps).
dust with powdered sugar before serving- this is just a classic lemon bar tradition! Use a sifter to dust the top of the lemon bars with some powdered sugar.