Prepare a 10 x 20 cm baking tin and line it with baking paper.

Chop the walnuts and dark chocolate and set aside.

Place the peeled bananas in a large bowl and thoroughly mash them with a fork. Add the egg, melted butter, sugar, and mix until you achieve a uniform consistency. Then stir in the plain flour and baking soda. Mix everything until you get a fairly thick batter.

Divide the prepared batter into two parts. To one part, add 2 tablespoons of cocoa powder, and to the other, mix in half of the chopped chocolate and nuts (25 g of each).

Pour half of the light colour batter into the previously prepared tin, then spoon half of the darker mixture on top. Repeat this process. Finally, sprinkle the remaining chopped chocolate and nuts over the top.

Bake the cake for about 40 minutes, or until a skewer inserted into the centre comes out clean. Enjoy!