Fill a 12-cup muffin tin with paper muffin liners. Set aside.
In a large mixing bowl, add mashed bananas, butter, sour cream, and sugar. Mix until combined, then add the eggs and vanilla, and mix again until well combined.
To the same bowl, add the flour, cocoa powder, baking powder, baking soda, salt, and chocolate chips. Mix everything until just combined; try not to overmix. Once combined, slowly pour in the milk a little bit at a time and mix after each addition until smooth.
Once combined, cover the bowl lightly with plastic wrap and refrigerate for 30 minutes to let the batter absorb moisture and thicken.
Preheat the oven to 425°F. Remove the bowl from the fridge and divide the batter evenly among the 12 cups in the prepared muffin tin. Optionally, you can sprinkle a few extra chocolate chips on top of each muffin before baking.
Bake for 7 minutes at 425°F. After 7 minutes, lower the temperature to 350°F and bake for an additional 20 minutes. The quick initial burst of heat is what creates a domed muffin top. After 20 minutes, or once the tops of the muffins spring back when gently pressed, you can remove them from the oven and allow them to cool in the pan for a few minutes.
Remove them from the pan and transfer them to a wire rack to cool completely. Serve once cooled and enjoy!