Preheat oven to 350F
Line two muffin trays with muffin liners.
Stir together the flour, cocoa powder, baking soda and salt in a small bowl and set aside.
Using a mixer, combine the butter and brown sugar for 2 minutes until light and well combined.
Add in bananas the add the egg, vanilla and sour cream. Mix thoroughly.
Add in the dry ingredients and combine fully, then add the chocolate chips.
Scoop 1/4 cup portions of the batter into the muffin liners. Top with more chocolate chips, as desired.
Bake for 20-25 minutes, until an inserted toothpick comes out clean and tops bounce back when tapped.