Preheat oven to 350 degrees F and grease a mini muffin tin.
Mash the banana well.
Stir together the wet ingredients, including the mashed banana
Stir together the dry ingredients in a separate bowl.
(Or, to do this step with a blender, place the bananas, eggs, butter, honey, and vanilla into a blender. Add the spinach if you plan to use it. Blend well.)
Stir the oats, flour, cocoa powder, baking powder, baking soda, salt, and chocolate chips, if using, to the wet ingredients
Fill prepared muffin tin about to the edge of each cup.
Bake for 10-12 minutes or until a cake tester inserted into the center comes out cleanly. Use a paring knife to remove from the pan and cool on a wire rack.
Serve at room temperature, slightly warm, or chilled.