Preheat Oven and Prep Muffin Cups:
Set oven to 350°F (175°C). Line 4 muffin cups with paper liners or grease them.Mash Banana and Mix Wet Ingredients:
In a medium bowl, fully crush the banana using a fork or potato masher. Mix in sugar, egg white, and oil with a spoon until barely combined (some lumps remain).Combine Dry Ingredients and Fold:
Add flour, baking powder, baking soda, salt, cinnamon, and nutmeg; stir until just incorporated. Optionally fold in chocolate chips.Fill Cups and Bake Muffins:
Evenly distribute batter among the 4 cups, filling each roughly two-thirds full. Bake for 18–25 minutes until a toothpick inserted in the middle comes out mostly clean and tops are golden.Cool and Serve Muffins:
Let muffins cool in the pan until safe to touch, then move to a wire rack to cool entirely. Serve.