Put the milk, butter / margarine, sugar and cocoa in a saucepan and bring to a boil. From the resulting composition, stop 15 tablespoons in which to add the melting chocolate (for decoration). The rest of the composition is cooled.
Over the beaten egg whites, add the yolks one by one, the quenched baking powder, the flour, the rum essence, the cooled composition and the walnuts. Bake in the oven for about 30 minutes. Pour melted chocolate over the baked cake.