1. Preheat your oven to 400 F (200 C). Line a 12-cup muffin tin with paper liners or grease it thoroughly with butter or non-stick spray.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until well combined.

3. In a separate medium bowl, whisk together the sourdough discard, milk, melted butter, large egg, and vanilla extract until well combined and smooth.

4. Pour the wet ingredient mixture into the bowl with the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Do not overmix; a few lumps are fine.

5. Divide the batter evenly among the 12 prepared muffin cups, filling each cup about two-thirds full.

6. Place the muffin tin in the preheated oven. Bake for 5 minutes at 400 F (200 C), then reduce the oven temperature to 375 F (190 C) and continue baking for another 15-20 minutes, or until a wooden skewer or toothpick inserted into the center of a muffin comes out clean.

7. Once baked, remove the muffin tin from the oven and let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely. Serve warm or at room temperature.