Place all dry ingredients in a medium bowl and whisk with a fork to combine very well. Place the bananas in a large bowl and mash with a fork or potato masher until there are no large pieces left. Add the sugar, egg, oil, vanilla and beat until well combined. Add the shredded zucchini and stir until incorporated. Add the dry ingredients and fold gently with a spatula until JUST combined. If you still see large pockets of flour mix just a LITTLE bit more and then stop. This step is crucial if you want light and fluffy muffins.