Step 1: Prepare the Crust

1. Preheat oven to 350°F (175°C). Line a 9×13-inch baking dish with parchment paper, leaving an overhang for easy removal.
2. In a mixing bowl, beat butter and sugar until light and creamy.
3. Mix in sourdough discard until fully incorporated.
4. Add flour and salt, mixing just until a dough forms.
5. Press dough evenly into the bottom of the prepared baking dish.
6. Bake for 20–22 minutes, or until edges are lightly golden. Remove and let cool slightly.

Step 2: Make the Lemon Filling

1. In a large bowl, whisk together sugar and flour.
2. Add eggs, lemon juice, lemon zest, and vanilla extract. Whisk until smooth.
3. Pour lemon filling over the warm crust.

Step 3: Bake and Cool

1. Return to the oven and bake for another 20–25 minutes, until filling is set and no longer jiggly.
2. Remove and let cool completely at room temperature.
3. Transfer to the refrigerator for at least 2 hours to firm up.

Step 4: Slice and Serve

1. Use parchment overhang to lift bars out of the pan.
2. Dust with powdered sugar and cut into squares or rectangles.
3. Serve chilled or at room temperature.