Preheat the oven to 375°F (190°C). Line a 12-cavity muffin pan with paper liners or grease with a non-stick cooking spray.
Whisk together the flour, cocoa powder, baking soda, and salt in a large bowl. Make a well in the center of the mixture. Set aside.
In a separate bowl, mix the mashed bananas, sugar, brown sugar, egg, and melted butter.
Add the banana mixture to the well in the flour mixture, and stir just until moistened. Fold in the chocolate chips.
Divide the batter evenly among the prepared muffin cups, using about 3 tablespoons of batter per cup.
Bake for 16-20 minutes or until a pick inserted into the center comes out clean.
Place the pan on a wire rack, and allow the muffins to cool in the pan for 5 minutes. Then remove the muffins from the pan to a wire rack to cool completely.
Notes
Store in an airtight container at room temperature up to 3 days.