In a large bowl, mash the banana with a fork until smooth.

Add the oats, cocoa powder, baking powder, nut butter, and egg (leave the chocolate chips out for now).

Mix well to form a thick batter, then fold in most of the chocolate chips, saving a few for the top.

Spoon the batter into 4 lined muffin cups.

Bake at 350°F (180°C) for about 17 minutes, or until set in the center.

Let cool slightly before serving.