Preheat the oven to 350. Grease two 8x4 pans.
In a small bowl, mash bananas (or, puree the bananas in a food processor). Set aside.
In a large bowl, cream butter, sugar, and brown sugar together until light and fluffy.
Add in the eggs, one at a time, beating well after each addition. Add in vanilla extract and mix until combined.
Add in flour, cocoa powder, salt, and baking soda, then mix until combined.
Add in mashed bananas, mixing until incorporated.
Divide the batter evenly between the two prepared loaf pans.
Bake for 35-40 minutes, or until a cake tester comes out clean with just a few moist crumbs stuck to it. (If the outside of the bread is cooking faster than the inside of the bread, tent a piece of foil over the bread while it finishes cooking).
Allow to cool for 10 minutes. Move to a wire cooling rack.