Preheat your oven to 350°F (175°C). This ensures that the muffins bake evenly and rise properly.

Grease a muffin tin with non-stick spray or line it with muffin liners to prevent sticking.

In a large mixing bowl, whisk together the all-purpose flour, whole wheat flour, baking soda, baking powder, salt, and ground cinnamon. Mixing these ingredients first helps to evenly distribute the leavening agents and spices throughout the batter.

In another bowl, combine the mashed bananas, grated zucchini, granulated sugar, brown sugar, vegetable oil, eggs, and vanilla extract. Mix until well combined. The bananas and zucchini add moisture and natural sweetness to the muffins.

Pour the wet ingredients into the dry ingredients. Gently fold the mixture with a spatula until just combined. Be careful not to overmix, as this can make the muffins tough.

If using, fold in the chopped walnuts and/or chocolate chips.

Spoon the batter into the prepared muffin tin, filling each cup about ⅔ full. This allows room for the muffins to rise without overflowing.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean. This indicates that the muffins are fully cooked.

Once baked, remove the muffins from the oven and allow them to cool in the tin for about 5 minutes. Then transfer them to a wire rack to cool completely. This prevents them from becoming soggy.

Enjoy your banana zucchini muffins warm or at room temperature. They can be stored in an airtight container for up to 3 days or frozen for longer storage.