Mix the dry ingredients - flour, cocoa powder, sugar, baking powder, baking soda and salt. Set aside.
Mix the wet ingredients - melted butter, eggs, vanilla extract, milk, and mashed banana.
Pour the wet ingredients into the dry ingredients and fold together until combined. Add the chocolate and fold together until fully incorporated. Cover & let the batter rest for 1 hour.
Line a muffin tin with cups leaving every other one empty. Fill the cups with the batter all the way to the top and then some. Sprinkle with more chocolate if desired.
Bake for 8 minutes at 425 degrees Fahrenheit, then drop the temperature to 350 degrees Fahrenheit and bake for another 8-10 minutes for regular sized muffins and 18-20 minutes for jumbo muffins. Let cool in the pan. Enjoy!