1. Preheat your oven to 375 F (190 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, unsweetened cocoa powder, baking soda, and salt until no lumps remain and ingredients are evenly distributed.

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 ingredients 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. Gently fold in the chocolate chips until they are evenly distributed throughout the batter. You can reserve a few to sprinkle on top of each muffin before baking.

6. Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. If you reserved chocolate chips, sprinkle them on top now.

7. Bake for 18-22 minutes, or until a wooden skewer or toothpick inserted into the center of a muffin comes out with moist crumbs, but not wet batter. The tops should be set and spring back lightly when touched.

8. Remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. Enjoy warm or at room temperature.