1. Preheat & Prepare: Set your oven to 350°F (175°C) and grease or line a 9×5-inch loaf pan with parchment.
2. Mix the Dry Ingredients: In a medium bowl, whisk together flour, cinnamon, baking soda, baking powder, and salt.
3. Cream the Wet Ingredients: In a stand mixer or bowl, beat melted butter and brown sugar for 5 minutes until light and fluffy.
4. Add Remaining Wet Ingredients: Stir in eggs, vanilla, sourdough starter, and zucchini.
5. Combine: Fold the dry mixture into the wet ingredients until just combined—avoid overmixing.
6. Add Extras: Stir in nuts, chocolate chips, or other mix-ins if desired.
7. Bake: Pour the batter into the loaf pan and bake for 50–60 minutes, checking at 50 minutes with a toothpick. If the top browns too quickly, tent with parchment or foil.
8. Cool: Let the bread rest in the pan for at least 1 hour before slicing.

Muffin Option: Scoop batter into lined muffin tins and bake for 15–20 minutes.

Long Fermentation: Mix sourdough starter, flour, eggs, and butter. Cover and ferment at room temperature for 6 hours, then refrigerate for 6–18 hours. Add the rest of the ingredients and bake as directed.