Preheat the oven to 180°C (approximately 350°F).
Coarsely chop the chocolate and melt with the butter over a hot, not boiling, water bath.
Beat the eggs and sugar until fluffy. Gradually stir in the slightly cooled chocolate mixture. Sprinkle in the flour and nuts. Fold together gently.
Spread the batter in a buttered baking dish and bake for about 30 minutes.
Remove the brownies from the oven and let cool. Serve dusted with powdered sugar.