1. Preheat your oven to 350 F (175 C). Grease and flour a 9×13 inch baking pan, or spray it well with non-stick baking spray.

2. In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set this bowl aside.

3. In a large mixing bowl, using an electric mixer, cream together the softened unsalted butter and granulated sugar until the mixture is light and fluffy, about 2-3 minutes.

4. Beat in the eggs one at a time, mixing well after each addition until fully incorporated. Stir in the vanilla extract and the sourdough discard.

5. Gradually add the dry ingredient mixture to the wet ingredients, alternating with the milk. Begin and end with the dry ingredients. Mix only until just combined; avoid overmixing.

6. Gently fold in the lemon zest.

7. Pour the batter evenly into your prepared baking pan. Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The cake should be golden brown on top and spring back lightly when touched.

8. While the cake cools slightly in the pan (about 10-15 minutes), prepare the lemon glaze. In a small bowl, whisk together the powdered sugar and fresh lemon juice until smooth. Start with 3 tablespoons of lemon juice and add more a tiny bit at a time until you reach your desired consistency.

9. Once the cake has cooled slightly, drizzle the glaze evenly over the top. Let the glaze set before slicing and serving.