Preheat the oven to 350 degrees F. Butter or spray the 12 molds in a regular-size muffin pan or fit with paper muffin cups. Place the muffin pan on a baking sheet.
In a large bowl, mix the flour, sugar, baking powder, baking soda, cocoa and salt. Melt the butter and half the chopped chocolate together in a bowl over a saucepan of simmering water. Remove from the heat.
In a separate mixing bowl, combine the mashed bananas, egg, melted butter mixture and milk until combined.
Pour the liquid ingredients over the dry ingredients and, with the a rubber spatula, gently stir to blend. Do not over mix the batter. Stir in the remaining chopped chocolate. Divide the batter evenly among the muffin cups filling each about ¾ full.
Bake the muffins until a knife inserted into the center of the muffins comes out clean, about 25 minutes. Transfer the pan to a rack and cool for 10 minutes. Remove from the muffin tins.