Preheat oven 350 degrees. Prepare mini muffin pan with cupcake liners or a generous layer of cooking spray then set it aside.
Beat the butter and sugars in a large mixing bowl until creamed. Then add egg, vanilla, banana and sour cream; continue to mix until well combined. Next add flour, cocoa and baking soda and stir until batter is blended.
Add a heaping tablespoon of batter to each muffin tin then sprinkle chocolate chips over the tops of muffins.
Bake mini muffins for 14-15 minutes at 350 degrees then remove the pan from the oven and allow it to rest on a cooling rack. Enjoy!
If using a standard size muffin tin; bake for 18-20 minutes.