Preheat oven to 350℉ and line a 9" loaf pan with parchment paper.
Whisk together dry ingredients in a bowl and set aside.
In a stand mixer, beat room temperature butter and brown sugar two minutes until light and fluffy.
Add in eggs, beating well after each addition.
Add honey, vanilla, sourdough starter and chopped bananas.
Slowly fold in the dry ingredients and zucchini and mix until just combined.
Pour into loaf pan and bake for about 60-65 minutes, or until the internal temperature at the center of the bread reaches 200°F with a thermometer.
Leave the bread in the pan for 10-15 minutes before inverting it onto a wire rack to cool.
This gives the bread a chance to set slightly, preventing it from falling apart when you invert it. Let it cool completely for about an hour before slicing. This prevents crumbling.