Preheat oven to 350° F and line two muffin trays with muffin liners.
In a small bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.
In a large bowl, use an electric mixer to beat the butter and brown sugar for 2 minutes until light and well combined. Add the mashed bananas, egg, vanilla, and sour cream. Mix thoroughly.
Slowly add the dry ingredients to the wet ingredients, stirring until combined. Fold in 1 cup chocolate chips.
Divide the batter evenly into the muffin cups. Sprinkle the remaining chocolate chips on the tops of the muffins.
Bake for 22-25 minutes, until an inserted toothpick or cake tester comes out almost entirely clean and tops bounce back when tapped.
Let cool in the muffin pan for 8-10 minutes, and then transfer to a wire rack to continue cooling.
Notes
Store in an airtight container at room temperature for up to 5 days.