Preheat the oven to 350 degrees Fahrenheit and coat a 9-inch loaf pan with butter or oil. Line the pan with parchment paper.
In a large or medium bowl mash bananas with a fork or a potato masher.
Add melted butter (or coconut oil), eggs and vanilla. Stir to mix.
In a separate bowl whisk together all dry ingredients: flour, brown sugar, baking powder, baking soda and salt.
Add whisked dry ingredients to the bowl with wet ingredients and stir to combine (don't overmix the batter).
Fold in most of the chopped dark chocolate (save 1-2 tablespoons for the top, if you want).
Transfer the batter to the prepared loaf tin. Top with remaining dark chocolate.
Bake for 50-55 minutes in the oven preheated to 350 degrees Fahrenheit or until a toothpick inserted in the middle of the bread comes out dry.
Cool on a rack for at least 30 minutes before slicing and serving.