Preheat oven to 350 degrees.
Lightly grease a 9×5 loaf pan with butter.
Combine flour, cinnamon, baking soda, baking powder, and salt well in a medium bowl.
In a stand mixer or bowl, mix melted butter (or melted coconut oil) and brown sugar for about five minutes or until light and fluffy.
Add in eggs, vanilla, sourdough starter, and grated zucchini.
Fold dry ingredients into the wet ingredients a little bit at a time until just incorporated.
Gently fold in any other add-ins you would like. Pecans, walnuts, and chocolate chips for example.
Pour into a greased pan. You could also line your loaf pan with parchment paper.
Bake for 50-60 minutes. Test bread at 50 minutes with a toothpick or cake tester.
Allow to cool in the pan for one hour before slicing.
