1. Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or lightly grease each cup with non-stick spray.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking soda, and salt. Make sure there are no lumps and the ingredients are evenly distributed.

3. In a separate medium bowl, mash the ripe bananas until mostly smooth with a few small chunks remaining for texture. Add the sourdough starter, melted butter, egg, and vanilla extract to the mashed bananas. Whisk until well combined and smooth.

4. Pour the wet banana mixture into the bowl with the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few streaks of flour are perfectly fine.

5. Gently fold in the semi-sweet chocolate chips until they are evenly distributed throughout the batter.

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

7. Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until a wooden skewer or toothpick inserted into the center of a muffin comes out clean. The tops should be golden brown and spring back when lightly touched.

8. Once baked, 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 them warm or at room temperature.