Preheat oven to 425. Line a muffin tin with muffin liners or spray with cooking spray. Set aside.
To grate the zucchini, cut off both ends and grate on a box grater. Gather the grated zucchini in a kitchen towel and squeeze out excess moisture. Set aside.
In one bowl, mash the bananas. Add the butter, maple syrup, yogurt, milk, eggs, vanilla extract, mix well. Stir in zucchini and mix until combined.
In another bowl whisk together the flour, cinnamon, salt, baking powder and baking soda.
Add the dry ingredients to the wet ingredients. Stir until just mixed. DO NOT over mix or your muffins will be tough. A few lumps is fine.
Divide the batter between the 12 prepared muffin cups.
Bake muffins at 425 degrees F for 5 minutes, then without opening the oven, reduce heat to 350 degrees F.
Bake at 350 for another 20-22 more minutes, or until a toothpick comes out clean. Baking them at the higher temperature at first helps them rise and gives a nice rounded top.
Remove from oven and remove muffins from the muffin tin as soon as possible. Allow to cool completely on a wire rack. Store in an airtight container for up to 3 days, or individually wrap and freeze for up to one month.
Notes
Add up to 1/4 cup more of maple syrup if you prefer a sweeter muffin. Oil can be subbed for butter if desired.