Preheat the oven to 180C/160C Fan/Gas 4. Grease and line a loaf tin.
Sift the plain flour, cocoa powder, baking powder, bicarbonate of soda, salt and sugar into a large bowl and mix until well combined.
Place the bananas in a separate bowl, stir in the eggs and melted butter.
Fold the banana mixture into the flour mixture until just combined. Fold in the chocolate and nuts and spoon the mixture into the loaf tin.
Bake for 50–60 minutes, or until the cake is risen and a skewer inserted in the centre comes out clean. Remove the cake from the oven, cool on a wire rack for 10 minutes, then remove the cake from the tin. Serve in slices.