Preheat the oven to 350 °F. Line a 9 x 13 baking pan with parchment paper.
Mix the flour, sugar, and salt together until combined.
Add in your cubed butter and cut it into the flour.*
Pour the mixture into the baking pan and press down firmly.
Bake for 25 minutes.
For the Lemon Filling
Reduce the oven temperature to 325 °F. Mix the sugar and flour together.
Whisk the sugar mixture with the eggs until well-combined.
Whisk in the lemon juice, and vanilla if using.
Pour the lemon mixture into the baking pan over the hot shortbread.
Bake for 20 - 25 minutes, or until the sides are set and the center is slightly jiggly, but not liquid.
Cool to room temperature then chill in the refrigerator.
Notes
You can do cut in the butter by pulsing the ingredients in a food processor 4 - 5 times, using a pastry cutter, or rubbing the butter into the flour by hand.