Preheat oven to 220 degrees C. Lightly grease a 12-cup muffin tin.
In a medium bowl, blend the banana, oil, egg and vanilla together.
In a large bowl, combine the flour, sugar, cocoa, baking powder and salt. Stir in the banana mixture until just blended. Fold in the chocolate chips. Spoon the batter into the prepared muffin tin, filling 3/4 full.
Bake in the preheated oven for 15 to 20 minutes. Remove muffins to a wire rack to cool completely.