Place the oven rack in the middle, preheat the oven to 350 F, and generously grease a 9 by 5-inch loaf pan. Set it aside.
Mash the bananas in a large bowl until smooth. The bananas should be overripe, with plenty of brown spots for maximum sweetness. Set it aside.
In a bowl, add the dry ingredients and mix using a wire whisk.
Ins a separated bowl Add the oil, sugar, egg, and vanilla extract and sourdough discard. Stir well until everything is smoothly incorporated. Add the mashed bananas. Sift in all-purpose flour, baking soda, salt, and cocoa powder. Gently fold the dry ingredients into the wet ingredients, careful not to overmix. Add half of the chocolate chips and gently mix the batter. Transfer to a prepared pan. Top with the remaining chocolate chips.
Bake in an oven for fifty minutes or until the cake tester comes clean.