1. 1. Prepare Your Baking Pan: Preheat your oven to 350 F (175 C). Grease and flour a 9×13 inch baking pan, or line it with parchment paper, leaving an overhang on the sides to easily lift the cake out later. This prevents sticking and ensures a clean release.

2. 2. Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Make sure there are no lumps and the ingredients are evenly distributed. Set this aside.

3. 3. Cream Butter and Sugar: In a large mixing bowl, using an electric mixer (handheld or stand mixer), cream together the softened unsalted butter and granulated sugar until the mixture is light, fluffy, and pale yellow. This usually takes about 2-3 minutes.

4. 4. Add Wet Ingredients: Beat in the eggs one at a time, mixing well after each addition until fully incorporated. Stir in the vanilla extract, sourdough discard, milk, lemon zest, and lemon juice. The mixture might look a little curdled at this stage due to the lemon juice interacting with the milk, but don’t worry, it will come together.

5. 5. Combine Wet and Dry: Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; overmixing can lead to a tough cake. A few small streaks of flour are acceptable.

6. 6. Bake the Cake: Pour the batter evenly into your prepared 9×13 inch baking pan. Bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The top should be golden brown and spring back lightly when touched.

7. 7. Cool and Glaze: Let the cake cool in the pan on a wire rack for at least 15-20 minutes before preparing the glaze. While the cake cools, whisk together the powdered sugar and lemon juice for the glaze in a small bowl until smooth. You want a pourable but not too thin consistency; add more lemon juice a teaspoon at a time if it’s too thick, or a little more powdered sugar if it’s too thin.

8. 8. Finish and Serve: Once the cake is mostly cooled (it can still be slightly warm), drizzle the lemon glaze evenly over the top. Allow the glaze to set for a few minutes before slicing and serving.