Chop 200 g dark chocolate and melt over a warm water bath. Cream 250 g butter, sugar and salt with the whisk of the hand mixer. Stir in the eggs one after the other. Stir in melted chocolate and cocoa. Mix flour and baking powder and stir in alternately with 150 ml cola.
Grease a ring cake tin (approx. 22 cm Ø, 2 litres capacity) and dust with flour. Put the dough and marshmallows in layers in the mould so that the marshmallows do not touch the rim. Fill in the last layer of dough and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 45-50 minutes. Take out and let cool down for about 15 minutes. Then carefully turn out onto a cake plate and let it cool down for about 45 minutes.
Coarsely chop the remaining dark and milk chocolate. Put 15 g butter and 30 ml cola in a small saucepan and heat until the butter has melted. Remove from the heat, add the chocolate and stir. Put the mixture into a mixing bowl. Sift the icing sugar and gradually stir it in with the whisk of the hand mixer. Spread the icing on the cake and chill for about 30 minutes.