(*) For a baking tray of about 21×37 cm (makes about 28 pieces) spread baking tray with parchment paper. Preheat the oven to 180 degrees. Remove the seeds from the sour cherries. Cream butter with sugar and a pinch of salt. Add eggs one by one until creamy. Add cocoa, grated dark chocolate, flour and baking powder and fold in. Fold in the sour cherries. Spread the dough evenly on the baking sheet. Bake in the middle of the oven for about half an hour.
Cool and cut into 5×5 cm cubes. Sprinkle with powdered sugar before serving.