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. This dry mixture will be easier to incorporate if it's already combined, preventing lumps from the protein powder.

In a large bowl, mash the 3 ripe bananas until mostly smooth (small lumps are fine—they add texture). Add the egg, Greek yogurt, maple syrup, and vanilla extract to the mashed bananas and whisk together until well combined. I like to whisk rather than stir here because it incorporates air and creates a lighter crumb in the finished muffins.

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—don't overmix, as this can develop gluten and make the muffins tough. The batter should look slightly lumpy and thick. Fold in the dark chocolate chips last, distributing them evenly throughout the batter.

Divide the batter evenly among the 12 muffin cups, filling each about three-quarters full. For even baking, I use an ice cream scoop to portion the batter—it ensures consistent sizing and helps the muffins bake evenly. Place the muffin pan in the preheated 350°F oven and bake for 20–25 minutes, until a toothpick inserted into the center of a muffin comes out with just a few moist crumbs (not wet batter).

Remove the muffin pan from the oven and let the muffins cool in the pan for 10 minutes. This resting time allows the crumb to set so they hold together when you remove them. After 10 minutes, turn the muffins out onto a wire rack to cool completely, or serve them warm if preferred.