Line a 9x9 square baking pan with parchment paper.

In a bowl, cream the butter and sugar until light and fluffy, about 3-5 minutes.

Mix in the sourdough discard, vanilla, and salt until just combined, then add the flour. Mix until the dough looks chunky like wet sand. It should press together easily and be soft.

Press the dough evenly into the prepared pan with your fingers.

Poke the dough with a fork all over and then bake for 25-35 minutes, or until lightly golden at the edges and set in the center.

While the dough is baking, make the filling.

In a bowl, rub the sugar and lemon zest together until the sugar is yellow in color and well scented with the lemon zest.

Add in the eggs whisk together until smooth.

Add in the sourdough discard and whisk until smooth.

Add fresh lemon juice, then whisk in until fully incorporated. The filling should be smooth and pourable and liquidy.

Pour the lemon mixture over the crust as soon as you pull it from the oven.

Bake the bars until the lemon filling is set and doesn't jiggle in the middle when you wiggle the pan, about 30-35 minutes.

Allow to cool in the pan at room temperature until it is room temperature (about one hour), and then move to the fridge to chill fully, at least two hours or overnight.

Before serving, dust the tops of the lemon bars with powdered sugar.