Preheat oven to 350 degrees F. Spray a muffin tin (or mini muffin tin) with non-stick cooking spray and set aside.

In a blender, puree bananas, milk, melted butter, spinach, and vanilla until smooth. Scrape down the sides of the blender and make sure that there aren't any stray spinach leaves! You want it nice and smooth!

Add brown sugar and egg to the blender. Mix until combined.

Add flour, baking powder, cinnamon, baking soda, and salt to the blender. Pulse until just combined and a smooth batter forms -- be careful not to overmix or you will have tough muffins.

Scoop batter into prepared muffin cups.

Bake mini muffins for about 15 minutes. If you make standard-size muffins, increase the baking time to 18-20 minutes.

Cool on a wire rack.