Preheat oven to 350°F (175°C).
Lightly grease a standard 12-cup muffin pan.
Mash the ripe banana thoroughly in a large bowl.
Add pumpkin puree and stir.
Whisk in the eggs.
Add milk and maple syrup.
Whisk in oil and vanilla.
Add cinnamon, pumpkin pie spice, baking powder, and salt.
Stir in the rolled oats.
Mix thoroughly until the oats are evenly coated.
Let the mixture rest for 5 minutes.
Fold in most of the mini chocolate chips if using.
Stir the mixture again before portioning.
Divide evenly among the 12 prepared muffin wells, making sure each receives both oats and liquid.
Sprinkle remaining chocolate chips over the tops if desired.
Bake for 24–30 minutes, until the tops and centers are set.
Cool in the pan for 10 minutes.
Carefully remove the oatmeal cups.
Transfer to a wire rack and cool further before storing or serving.