Grease a rectangular-shaped cake mould with butter. Place in the freezer. Beat the butter with the sugar and the vanilla sugar until the sugar dissolves. Add the eggs, one by one. Keep on mixing. Sieve the flour into the bowl. Keep on mixing and add the yogurt and lemon juice. Melt the chocolate and mix it with half of the dough. Mix this dough with the other half. Bake it in a preheated oven at 180°C for 50 minutes.