Preheat oven to 350 F. Line a muffin tin with 12 paper liners.
In a 2 cup measuring cup, place the banana and mash with a fork until you get a puréed consistency.
Finely grate the zucchini with a cheese grater. Wrap in paper towel or kitchen towel and squeeze the excess liquid out. You can leave some liquid so it is moist and damp still. Not completely wrung out.
In a medium mixing bowl, add the puréed banana, milk, oil, brown sugar, and vanilla extract. Mix well until combined. Next add in the finely grated and squeezed zucchini and mix well again until zucchini is evenly distributed in the batter.
To the batter, add the flour, baking powder, baking soda, salt and cinnamon in this order. Try to mix the dry ingredients on top of the batter before incorporating. Then gently mix all together until no dry streaks go through it. Try not to over mix.
Scoop into prepared muffins tin and fill about 3/4 way full. Bake 18-24 minutes until tooth pick comes out clean and tops spring back when gently tapped.
Allow to cool for a couple minutes in the muffin tin, then remove to a wire rack. Cool completely before eating (if you can wait!- they taste better this way imo) and freeze leftover muffins for 2-3 months.