Preheat oven to 180°C/160°C fan-forced. Lightly grease a square 20cm cake tin
Place butter and dark chocolate in a saucepan, over medium-low heat.
Cook, stirring, for 2 to 3 minutes or until smooth.
Remove from heat.
Stir in sugar and let stand for 5 minutes to cool slightly. Whisk in flour and eggs.
Spoon mixture into cake tray. Bake for 12 minutes or until firm. Stand for 5 minutes. Transfer to a wire rack to cool. Serve dusted with icing sugar.