Step 1. Preheat oven to 350 degrees.
Step 2. Prepare the baking dish with butter or parchment paper.
Step 3. In a large mixing bowl, combine the flour, cinnamon, baking soda baking powder, and salt.
Step 4. In a separate bowl, cream the butter and sugar until smooth and fluffy.
Step 5. To the creamed butter and sugar, add the eggs, vanilla, sourdough starter, chopped banana, and zucchini.
Step 6. Slowly add the dry ingredients to the wet ingredients until just incorporated. (Add in any mix-ins after the wet and dry ingredients are combined.)
Step 7. Pour batter into the prepared dish.
Step 8. Bake for 60 minutes.
Step 9. After you check for doneness, remove from oven and allow to cool completely. Then serve and enjoy!