Preheat oven to 350 degrees. Spray muffin pan with non-stick spray or put muffin liners in each cup.
Add the butter and sugars to a mixing bowl and mix until well combined.
Beat in the egg, vanilla, banana, and sour cream.
Stir in the flour, cocoa powder, baking soda, and salt.
Fill each muffin cup two-thirds full. Sprinkle the tops with chocolate chips and gently press them into the batter.
Bake for 20-25 mins, until risen, firm to the touch and a toothpick inserted in the middle comes out clean.
Let cool completely before serving.