Preheat the oven to 180 ° C top and bottom heat Preheat.
Chop dark chocolate coarsely and melt with butter over a hot water bath (not boiling).
Beat sugar with eggs until fluffy and gradually add to slightly cooled chocolate. Sift flour over mixture and add milk chocolate pieces. Mix batter.
Spread batter into a buttered baking dish and bake in preheated oven at 180°C (approximately 350°F) for about 30 minutes.
Remove from oven and cool. Cut into square pieces and serve dusted with powdered sugar.