Preheat the oven to 180°C.
Prepare a 10 x 20 cm loaf tin by lining the bottom and sides with parchment paper.
Chop the walnuts and dark chocolate and set aside.
Place the peeled bananas in a large bowl and mash them thoroughly with a fork. Add the egg, melted butter, and sugar, and mix until well combined. Then, add the flour, baking soda, and a pinch of salt. Mix everything together until you have a fairly thick batter.
Divide the batter into two equal parts. To one part, add 2 tablespoons of cocoa powder. To the other, fold in half of the chopped chocolate and walnuts (25 g each).
Spoon half of the light batter into the prepared pan, then top it with half of the dark batter. Repeat the process with the remaining batter. Finally, sprinkle the rest of the chopped chocolate and walnuts (25 g each) over the top.
Bake the cake for about 40 minutes, or until a skewer inserted into the centre comes out clean. Enjoy!