Preheat your oven to 350°F and grease a 12-cup muffin pan with cooking spray or butter, making sure to coat the bottom and sides evenly. While the oven heats, whisk together the rolled oats, vanilla protein powder, baking powder, cinnamon, salt, and nutmeg in a medium bowl.

In a large bowl, mash the 3 ripe bananas until mostly smooth. Add the egg, Greek yogurt, maple syrup, and vanilla extract to the mashed bananas and whisk together until well combined.

Pour the dry ingredient mixture from Step 1 into the wet banana mixture from Step 2 and fold gently with a spatula until just combined. Fold in the dark chocolate chips.

Divide the batter evenly among the 12 muffin cups, filling each about three-quarters full. Bake for 20–25 minutes, until a toothpick inserted into the center of a muffin comes out with just a few moist crumbs.

Remove the muffin pan from the oven and let the muffins cool in the pan for 10 minutes. Turn the muffins out onto a wire rack to cool completely, or serve them warm if preferred.