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, baking soda, salt, and ground cinnamon until evenly distributed.

3. In a separate medium bowl, mash the ripe bananas until mostly smooth. Add the sourdough starter discard, melted and cooled butter, large egg, and vanilla extract to the mashed bananas. Whisk these wet ingredients together until well combined.

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. If using chocolate chips or chopped nuts, gently fold them into the batter until evenly distributed.

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.