Preheat your oven to 375°F and grease a standard 12-cup muffin tin with butter or cooking spray. While the oven heats, finely shred the zucchini and squeeze it firmly in a clean kitchen towel to remove excess moisture—this is crucial because wet zucchini will make your muffins soggy. I always squeeze it twice to make sure I've removed as much liquid as possible, which gives you a better texture in the final muffin.

In a large bowl, whisk together the egg, mashed banana, sugar, vanilla extract, and melted butter until well combined and smooth. The mixture should look homogeneous with no visible lumps of banana remaining. Fold in the squeezed zucchini gently until evenly distributed throughout the wet mixture.

In a separate medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Make sure everything is evenly distributed so the leavening agents and cinnamon are spread throughout—this prevents pockets of bitterness from concentrated baking soda and ensures consistent flavor in every muffin.

Pour the dry ingredient mixture from Step 3 into the wet mixture from Step 2 and fold together gently with a spatula until just combined. Stop mixing as soon as you don't see any visible streaks of flour—overmixing develops gluten and creates tough, dense muffins. A few small lumps are perfectly fine and actually desirable.

Divide the batter evenly among the 12 greased muffin cups, filling each about three-quarters full. Bake in the preheated 375°F oven for 18-20 minutes, until a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs clinging to it. The tops should be lightly golden brown when finished.