Start by preheating your oven to 350ºF (175ºC). Line your muffin tin with paper liners or lightly grease the cups, this little step makes cleanup so much easier.
In a large bowl, whisk together the flour, sugars, baking soda, baking powder, cinnamon, and salt until everything is well mixed.
Use a separate bowl to smash the ripe bananas into a smooth texture. Then add the eggs, melted butter, buttermilk, and vanilla, whisking until the mixture is smooth and inviting.
Pour the wet mixture into the dry and carefully fold them together. Be careful not to overmix, it keeps your muffins tender. Next, shred the zucchini and squeeze out as much moisture as you can using a clean towel. Folding in the zucchini last helps keep your muffins moist without being soggy.
Scoop the batter into the muffin cups, filling each about three-quarters full so they have room to rise beautifully.
Bake for about 24 minutes, but start checking around 20 minutes by poking a toothpick in. If the tester is clean or has only a few crumbs, the muffins are baked just right.
Let them rest in the pan for five minutes to firm up, then transfer to a wire rack to cool before enjoying.