For the banana-orange cake, grease a loaf pan and dust with flour. Preheat oven to 175 °C. Cream butter and sugar in a food processor. Add eggs a little at a time.
Grate the zest from one orange and squeeze all the oranges. Peel the banana, mash with a fork and immediately stir into the egg mixture with the orange juice and zest.
Grate chocolate and mix with sifted flour, baking powder and nuts, then stir into the batter.
Pour the batter into the loaf pan and bake the banana-orange cake for about 1 hour (test with chopsticks).