Heat the oven to 350°F (175°C). Grease a 9 x 5-inch loaf pan and line it with parchment paper if desired, leaving a little overhang to make lifting the baked loaf easier.

Finely shred the zucchini, then lightly squeeze or blot it to remove excess liquid. Do not press it completely dry because some moisture helps keep the bread soft.

Whisk the flour, baking soda, baking powder, salt, and cinnamon together in a medium bowl. Mixing the dry ingredients first distributes the leaveners and spice evenly throughout the loaf.

In a large bowl, whisk the melted, slightly cooled butter and sugar together. Add the eggs and vanilla, mixing until the mixture is smooth and well combined.

Stir the mashed bananas and shredded zucchini into the butter mixture. Continue until the produce is evenly distributed, but there is no need to beat the batter.

Add the dry ingredients to the wet ingredients and fold just until no dry streaks remain. Fold in the walnuts, pecans, or chocolate chips if using, taking care not to overmix.

Pour the batter into the prepared loaf pan and smooth the top. Bake for 50–60 minutes, checking near the end with a toothpick inserted in the center.

The loaf is ready when the toothpick comes out clean or with a few moist crumbs. If the top browns too quickly, loosely cover it with foil during the final 15 minutes of baking.

Cool the bread in the pan for 10–15 minutes, then transfer it to a wire rack. Let it cool for at least another 20 minutes before slicing, or cool it completely for the neatest slices.