Preheat the oven to 350°F and grease 9 cavities of a muffin pan. Melt the coconut oil and set aside to cool to room temperature.
Fill a 1-cup measuring cup with oats and level the top with a knife. Blend the oats in a food processor or blender until they resemble a fine flour. If needed, stir and reblend to break up larger chunks. Set aside.
Mash very ripe bananas in a small bowl with the back of a fork. Mash thoroughly.
Add the mashed banana, egg, maple syrup, baking soda, baking powder, cinnamon, salt, melted coconut oil, and vanilla extract to a large bowl. Stir until well combined.
Add the blended oats and stir until combined. Add dark chocolate chips and mini chocolate chips and stir again.
Transfer the batter into the prepared muffin pan, adding equal amounts to the 9 cavities; cavities should be about 3/4 filled. If desired, add a few more chocolate chips on top.
Bake for 23–26 minutes or until lightly browned around the edges and no longer gooey/wet looking on top (if you lightly touch the muffin, it should spring back).
Remove from oven and let stand 5 minutes before using a fork to coax the muffins out onto a cooling rack.