• Preheat oven to 350℉.
• Line a 9x13 inch baking dish with parchment paper. (The edges of the parchment paper should come up the sides of the pan on two opposite sides. This will let you lift the lemon bars easily from the pan after baking). Set aside.
• In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and powdered sugar on medium speed.
• Add the sourdough starter, salt and vanilla and mix until combined.
• Set the stand mixer to low speed and slowly add the flour. Mix until just fully combined.
• Press the dough evenly into the bottom of the prepared baking pan to form the bottom layer of crust.
• Bake the crust for 20-23 minutes or until starting to turn golden brown.

Prepare the Lemon Filling

• In a large bowl, whisk together the filling ingredients (eggs, sugar, sourdough starter, lemon zest, lemon juice, and salt) until smooth.
• Pour the mixture over the crust while still warm from the oven.
• Reduce heat in oven to 325℉.
• Bake the lemon bars for 20-23 minutes or until the mixture is just fully set. It should wobble but not appear wet when done.
• Allow to cool in pan completely at room temperature (about 45 minutes).
• Use parchment paper to gently lift the bars from the pan. Dust with extra powdered sugar through a mesh sieve.
• Cut the bars into approximate 2 inch squares and serve.