Preheat the oven to 375º F (180° C). Line a pound cake pan of about 9.8 x 4 inches with high walls with parchment paper.
In a medium bowl, sift together the flour with the cocoa powder, baking soda and salt.
Separately, in a large bowl add the banana and mash it with a fork until you get a thick paste.
Add in the butter, oil, brown sugar, egg and vanilla extract. Mix with a hand whisk (or electric mixer) for 2 minutes until everything is well integrated.
Add the flour mixture and beat for 1 minute until smooth and lump-free. Optionally, chocolate chips can be added.
Pour the mixture into the pan and bake at 375º F (180° C) for approximately 50-55 minutes until a wooden toothpick comes out clean when inserted.
Once ready, let the banana bread rest for 10 minutes inside the pan. Then, turn it out and let it cool down completely on a wire rack.