1. Preheat your oven to 350 F (175 C). Lightly 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 bread out later.

2. In a large bowl, whisk together the melted butter and granulated sugar until well combined. Beat in the eggs one at a time, then stir in the vanilla extract and the unfed sourdough starter until smooth. The mixture should be creamy and well-integrated.

3. In a separate small bowl, mash your ripe bananas with a fork until mostly smooth, with a few small lumps remaining for texture. Add the mashed bananas to the wet ingredient mixture and stir until just combined.

4. In another medium bowl, whisk together the all-purpose flour, baking soda, salt, and unsweetened cocoa powder. Make sure there are no lumps of cocoa powder. This ensures even distribution of the dry ingredients.

5. Gradually add the dry ingredient mixture to the wet ingredient mixture, mixing until just combined. Be careful not to overmix; overmixing can lead to a tough bread. A few streaks of flour are okay.

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

7. Pour the batter into your prepared loaf pan and spread it evenly. Bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean or with moist crumbs attached, but no wet batter.

8. Let the banana bread cool in the pan for 10-15 minutes before carefully lifting it out onto a wire rack to cool completely. Slicing it while it’s still warm is tempting, but letting it cool fully allows the flavors to set and prevents it from crumbling.