Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper.
Mash the bananas in a large mixing bowl until mostly smooth.
Grate the zucchini, then squeeze out as much water as possible using a clean kitchen towel or paper towels.
Mix the wet ingredients: Add eggs, oil, vanilla, mashed banana, and zucchini into a bowl and stir until combined.
Whisk the dry ingredients in a separate bowl: flour, sugar, baking soda, cinnamon, and salt.
Combine wet and dry ingredients, stirring gently just until no dry flour remains. Don’t overmix.
Fold in add-ins if using—nuts, chocolate chips, etc.
Pour the batter into your prepared loaf pan and spread evenly.
Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean.
Cool the loaf in the pan for 10–15 minutes before transferring to a wire rack to cool completely.