Preheat oven to 350°F (175°C). Grease a 9x5 loaf pan or line with parchment paper.
Grate zucchini and lightly squeeze out excess moisture using a clean towel.
In a large bowl, whisk together sourdough starter, eggs, melted butter, honey, and vanilla.
Stir in flour, baking soda, baking powder, cinnamon, and salt until just combined.
Fold in zucchini and optional mix-ins. Do not overmix.
(Optional) Cover and let batter rest at room temperature for 2–4 hours for deeper flavor.
Pour batter into prepared loaf pan.
Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
Cool in pan for 10–15 minutes before transferring to a wire rack to cool completely.