For the banana cake, separate eggs. Cream butter with half the sugar and vanilla sugar. Mash bananas with yolks and stir into butter mixture.
Beat egg whites with remaining sugar until stiff peaks form. Mix one third of the beaten egg whites with the butter and gently fold in the remaining beaten egg whites, nuts and flour.
Pour the mixture into a buttered loaf pan and bake at medium heat for about 45 minutes. Then cover the banana cake with the chocolate icing.