Heat oven to 350F and line 9x12 pan with parchment paper
Whisk 2 1/2 cups of flour with powdered sugar and salt together.
Pour in the melted butter and mix until well combined. You can add a dash of vanilla as optional
Sprinkle the dough into the pan and press down into a flat layer. Bake it for 20 minutes
While the crust is baking, using a large bowl, add the sugar and zest of three lemons. Whisk until combined.
Continue to add the remaining of flour and mix well.
Pour in the lemon juice and add the eggs. Then mix very well until completely combined.
Pour the filling onto the warm crust and bake for another 25 minutes.
Remove from the oven and allow to cool for at least an hour.
Put in the refrigerator for at least 2 hours or overnight
When it's ready to serve, dust with powdered sugar and cut into 16 bars using a clean, sharp knife.