Preheat oven to 350 degrees. Butter and flour a 9x5-inch loaf pan.
In a large bowl, whisk together flour, cocoa powder, baking soda, and salt.
In separate large bowl, combine sugar, butter, buttermilk, vanilla, egg and egg yolk until smooth. Pour wet ingredients over dry and stir until combined, then fold in chocolate chips and mashed bananas.
Transfer batter to prepared baking dish and bake until deeply golden and a toothpick comes out clean, about 55-60 minutes. Allow to cool in the pan, over a cooling rack for 5-10 minutes.
Remove from pan and allow to cool completely. Slice and serve.