Mix sultanas and 1⁄8 l Guinness in a bowl. Cover and leave to stand for about 2 hours.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease a springform pan (18 cm Ø) and dust with flour. For the dough, grate the chocolate coating. Cream 150 g butter and 125 g sugar with the whisk of the mixer for about 5 minutes.
Stir in the eggs one by one. Mix 125 g flour, couverture, cocoa and baking powder. Stir into the butter-sugar cream. Stir in sultanas together with Guinness marinade. Smooth the dough in the tin.
Bake in a hot oven for about 1 hour. Cover with aluminium foil after about 30 minutes.
Take out the cake and spread 4 tbsp. Guinness while still hot. Let it cool down and remove from the springform pan. Mix cream stiffener and 1 tablespoon sugar. Whip the cream until stiff and let the sugar mixture trickle in.
Spread the cake loosely with the cream just before serving.