1- Preheat the oven to 350°F (175°C). Line a 9×13 inch baking dish with parchment paper.

2- Combine 2 1/2 cups flour, powdered sugar, and salt in a bowl. Pour in melted butter and mix until fully combined. Optionally, add a teaspoon of vanilla with the butter for added flavor.

3- Press the dough evenly into the lined baking dish and bake for about 20 minutes or until light golden.

4- While the crust bakes, combine sugar and lemon zest in a food processor; pulse until the sugar turns light yellow and the zest is fully incorporated. If no processor is available, finely mince the zest and mix it with the sugar by hand.

5- Mix the zest-infused sugar with the remaining 1/2 cup flour in a large bowl.

6- Add lemon juice and eggs to the sugar mixture, whisking thoroughly until fully combined.

7- Pour the lemon filling onto the warm crust. Place the baking dish on a larger baking sheet and bake for approximately 25 minutes at 350°F (175°C), rotating the pan halfway through baking.

8- Allow the bars to cool for one hour at room temperature, then chill in the refrigerator for at least two hours to set.

9- Dust with powdered sugar if desired. For clean cuts, use a sharp, damp knife, wiping and re-wetting the blade between each cut.