Preheat your oven to 350°F (175°C). Grease a 9×9-inch baking dish or line it with parchment paper, leaving an overhang on two sides for easy removal.

In a mixing bowl, combine the flour, sugar, softened butter, and salt. Mix until crumbly and well combined. Press the mixture evenly into the bottom of the prepared baking dish to form the crust.

Place the crust in the preheated oven and bake for 15-20 minutes, or until lightly golden brown.

While the crust is baking, prepare the filling. In a separate mixing bowl, whisk together the granulated sugar and flour. Add the eggs, sourdough discard, lemon zest, and lemon juice to the bowl. Whisk until smooth and well combined.

Pour the filling mixture over the partially baked crust. Return the baking dish to the oven and bake for an additional 20-25 minutes, or until the filling is set and the edges are lightly golden brown.

Remove the lemon bars from the oven and allow them to cool completely in the baking dish on a wire rack. Once cooled, transfer the baking dish to the refrigerator and chill the bars for at least 2 hours, or until firm.

Once chilled, use the parchment overhang to lift the lemon bars out of the baking dish. Place them on a cutting board and slice into squares or bars. Dust with powdered sugar before serving, if desired. Serve chilled or at room temperature, and enjoy these tangy and sweet sourdough discard lemon bars!