In a bowl, mix flour, cocoa powder, baking powder, baking soda, and salt until combined.
In a blender, blend milk, banana, and spinach until it resembles the texture of a smoothie and the spinach is finely incorporated.
In a mixing bowl, mix together egg, vanilla, butter, and blended ingredients.
Begin mixing the sugar into the wet ingredients until combined.
Slowly incorporate the dry ingredients into wet ingredients, until everything is combined.
Place your batter in a paper lined muffin tin, and bake at 350° F for 15 minutes or until a toothpick comes out clean.