Preheat the oven to 350 degrees F (175 degrees C). Line a12-cup muffin tin with paper liners.
Combine flour, cocoa, baking powder, baking soda, and salt together in a large bowl.
Whisk sugar, oil, milk, egg, and vanilla extract together in a separate bowl until smooth; stir into flour mixture until just moistened. Fold in bananas and chocolate chips.
Divide batter among the muffin cups, filling each about 3/4 full.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 20 to 25 minutes.