1. Preheat your oven to 350 F (175 C). Grease and flour a 9×5-inch loaf pan, or line it with parchment paper, leaving an overhang on the long sides to easily lift the loaf out later.

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

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

4. Beat in the eggs one at a time, mixing well after each addition. Stir in the sourdough discard, fresh lemon juice, lemon zest, and vanilla extract until just combined. The mixture might look a little curdled, but don’t worry, it will come together.

5. 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 streaks of flour are okay.

6. Pour the batter evenly into your prepared loaf pan. Bake for 45-55 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean. The top should be golden brown and spring back lightly when touched.

7. Let the loaf cool in the pan for about 10-15 minutes before carefully lifting it out onto a wire rack to cool completely. While the cake cools, prepare the glaze by whisking together the powdered sugar and lemon juice until smooth. Once the loaf is completely cool, drizzle the glaze evenly over the top.