Make the lemon curd:

1. In a medium sized saucepot, combine lemon zest, lemon juice, sugar, egg yolks, and salt. Whisk to combine. 
2. Place your pot onto a stove burner set to medium low heat. Cook the curd, whisking constantly, until it thickens enough to coat the back of a spoon and the consistency is similar to pudding.
3. This will take about 10 to 15 minutes (depending on the size of your pot and how powerful your stove burner is). Don't try to rush this step by using high heat as it can curdle the curd!
4. Add butter and whisk until melted and smooth. 
5. Strain the curd into a medium sized bowl. Set aside to prep the cookie dough.

Make the lemon cookie dough:

1. Preheat your oven to 350°F. Butter and line the bottom and sides of an 8x8 inch pan with parchment paper. Leave a bit of parchment paper hanging over the edges of the pan to make it easier to lift the bars out later. 
2. In a large bowl, combine butter, sugar, and lemon zest. Cream together until light and fluffy. 
3. Whisk eggs in, one at a time, beating well after each addition. Whisk in lemon juice and vanilla. The mixture may appear slightly curdled, but that's fine!
4. Add flour, baking powder, and salt. Fold together until you have a streak free, thick and sticky cookie dough. 
5. Spread about 2/3 of the cookie dough into the bottom of your baking pan and place the remaining dough in the fridge. Bake the 2/3 of the cookie dough in the center of your preheated oven for about 10 minutes or until the edges just start to lose their shine. The center will still appear wet.
6. Take the pan out of the oven and pour the lemon curd on top. Spread to even out.
7. Take the remaining cookie dough out of the fridge and crumble it over the curd. It won't fully cover the curd but try to get it evenly distributed on top.
8. Return the pan to the center of your oven and bake for 15 to 20 minutes more or until the edges are browned and the cookie crumble on top no longer feels wet if you gently poke it. The top will be a very pale yellow color and the browning will mostly be concentrated around the sides. 
9. Cool the bars completely to room temperature in the pan. Use the parchment paper overhang to pull the bars out and onto a cutting board. 
10. To slice, use a sharp serrated knife to gently saw back and forth to cut through the top cookie crumble then press down to cut through the curd and bottom cookie crust. Lightly dust the tops with powdered sugar if desired before serving!