Beat butter with sugar and eggs until fluffy.
Melt the dark chocolate and stir into the butter-egg mixture with the yogurt. Finally, sift in flour, cocoa and baking powder and fold in carefully.
Divide the mixture between two loaf pans and bake at 175 °C for about 40 minutes.
If you like, you can spread the cake with jam while it is still hot and cover it with chocolate icing.