Preheat oven to 350°F (175°C) and grease a 9x5 inch loaf pan.
In a large mixing bowl, mash the bananas with a fork or whisk until smooth.
Stir the melted butter into the mashed bananas.
Add the sugar, egg, and vanilla extract; mix until well combined.
Sift together the flour, cocoa powder, baking soda, and salt.
Gradually fold the dry ingredients into the wet ingredients until no streaks of flour remain.
Stir in the chocolate chips.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 55 to 65 minutes, or until a tester inserted into the center comes out clean.
Cool in the pan for 10 minutes before transferring to a wire rack to cool completely.