Preheat your oven to 350 degrees and, using paper cupcake liners, line six muffin cups with the liners. If you do not have cupcake liners, you can spray the muffin cups with cooking spray.
In a large bowl, mash the banana very well. You want to mash it thoroughly so that there are no lumps and the banana is in a smooth puree. You may use a food processor or a stick blender for this step.
Whisk the sugar into the banana, and then add the oil, egg, and vanilla. Stir very well until all of these ingredients are thoroughly combined.
Using the same mixing bowl, sprinkle the flour, cocoa, baking soda, and salt onto the top of the wet ingredients. Use a spoon to stir this mixture until the ingredients are fully incorporated.
Last of all, stir in the chocolate chips, reserving about a tablespoon of the chocolate chips.
Divide the batter among the muffin cups, and sprinkle the tops of the muffins with the reserved chocolate chips.
Bake the muffins for 18 to 20 minutes until the tops of the muffins spring back lightly when touched.
Let the muffins cool in the pan for about five minutes and then remove them to cool on a wire rack. Let them cool for about 20 minutes before serving them slightly warm.