Lightly grease a 9×5″ loaf pan with butter and a light dusting of flour, or line it with parchment paper. (Don't preheat the oven just yet)
In a large mixing bowl, add sourdough starter (fed or unfed), pumpkin puree, sour cream, egg, vanilla, melted butter, and brown sugar. Use a whisk to combine.
Then add in the flour, baking soda, baking powder, salt, and ground cinnamon. Whisk to combine just until smooth. Avoid over mixing.
Stir in any optional add-ins if desired (chopped nuts, shaved coconut, or chocolate chips).
At this point, you can either continue the recipe or you can place the batter into the fridge for an overnight ferment up to 24 hours. This will deepen the sourdough flavor and provide more gut health benefits. This is completely optional.
Pour the batter into the prepared loaf pan and smooth it out so it’s evenly distributed.
PLEASE READ CAREFULLY: Place the filled loaf pan into a turned-off oven and then preheat to 375 degrees Fahrenheit. Set the timer for 35 minutes after the oven has finished preheating. (see notes for reason)
When the bread has finished baking a toothpick should come out clean in the center. If it has residue, it needs 2-3 more minutes to finish baking.
Let it cool for 10 minutes in the pan on a wire cooling rack, then turn the bread out and let it cool the rest of the way. Enjoy!