Beat the butter and sugar until fluffy. Add the eggs and beat until pale yellow. Melt the chocolate over a pot of simmering water, then stir into the mixture. Mix the flour with the salt and baking powder. Fold into the egg mixture. Gradually pour the milk into the mixture to form a smooth batter.
Beat the cream with the sugar, vanilla sugar, and eggs.
Butter a deep baking sheet, pour in the batter and smooth. Spread the cream mixture and swirl gently with a fork. Bake in a preheated oven at 180°C (approximately 350°F) for 40 minutes, until golden brown. Check with a toothpick for doneness. Remove the finished cake from the oven and let cool. Cut into rectangles and serve chilled.