First mash the bananas. Ripe bananas are best for this. Finely chop the chocolate, mix with the cocoa powder and pour the hot coffee over it. Stir until the cocoa and chocolate have dissolved. (Even if it doesn`t matter if there are a few pieces of chocolate left over.) Mix with the banana. Stir in sugar, salt and margarine. Add the vanilla (I often use the Dr. Oetker vanilla grinder, but you can also use vanilla sugar and less normal sugar). Mix the flour and baking powder and stir in. Chop walnuts and add to the batter. I use a small baking dish as a form. These grease and flour. Pour in the dough and bake in the oven at 180 degrees top / bottom heat for about an hour.