Preheat oven to 180 C / Gas 4. Lightly grease a loaf tin.
Mash the bananas and combine with melted butter and egg yolks; mix well. Beat the egg whites until fluffy; add the sugar and continue beating until sugar has dissolved and mixture is stiffer.
In a mixing bowl, combine the flour and bicarbonate of soda; add the mashed bananas and mix well. Gently fold in the beaten egg whites. Add walnuts and chocolate; mix gently.
Transfer cake mixture to the loaf tin. Bake for 50 minutes in preheated oven. Let cool and garnish with prepared chocolate sauce and sliced bananas.