1. Prepare Your Baking Pan: Preheat your oven to 350 F (175 C). Lightly grease and flour a standard 9×5-inch loaf pan, or line it with parchment paper, leaving an overhang on the sides for easy removal. This prevents sticking and ensures a clean release.

2. Combine Wet Ingredients: In a large mixing bowl, whisk together the sourdough discard, melted butter, granulated sugar, egg, and vanilla extract until the mixture is smooth and well combined. Make sure there are no lumps of butter or egg.

3. Whisk Dry Ingredients: In a separate medium bowl, combine the all-purpose flour, baking powder, baking soda, and salt. Whisk these dry ingredients together thoroughly to ensure they are evenly distributed. This step is crucial for an even rise.

4. Combine Wet and Dry Mixtures: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix the batter; overmixing can lead to a tough loaf. A few small lumps are perfectly fine.

5. Add Milk and Final Mix: Pour in the milk and mix gently until the batter is just smooth. Again, avoid overmixing. The batter should be thick but pourable.

6. Transfer to Pan and Bake: Pour the batter into your prepared loaf pan, spreading it evenly with a spatula. Place the pan in the preheated oven.

7. Check for Doneness: Bake for 30-35 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 when lightly touched.

8. Cool Before Slicing: Once baked, remove the loaf from the oven and let it cool in the pan for about 10-15 minutes before carefully lifting it out onto a wire rack to cool completely. Slicing while warm can make the loaf crumbly.