Preheat the oven to 180℃/350℉. Line an 8x8-inch cake pan with parchment/butter paper and set aside.
To a medium-sized bowl add butter, chopped dark chocolate, and cocoa powder. Place the bowl on top of a medium-sized saucepan filled with hot water. Stir occasionally until completely melted and combined. Set aside. (You can also employ a microwave for this process).
In another bowl sift in the flour and salt. Add chopped milk chocolate, almonds, and walnuts. Mix well till everything is well coated in flour. Set aside.
In a large bowl add bananas and smash them with a fork. No need to be thorough.
Add sugar and egg. Beat on medium speed for 3-4 minutes or till everything is well combined and almost pale.
Into this add melted chocolate mixture. Combine with a spatula or mixer till incorporated.
Add flour mixture and mix with a spatula till well combined.
Pour the brownie mix into the prepared pan and sprinkle some chocolate chips or chunks on top(optional).
Bake in a preheated oven at 180℃/350℉ for 25-30 minutes or until a skewer comes with a few moist crumbs.
Remove from the oven and let it cool completely in the pan itself before cutting.