1.Preheat oven to 180°C (160°C fan-forced). Grease 1.5-litre (6-cup) ovenproof dish.
2.Heat butter and milk in ovenproof dish in microwave oven on HIGH (100%) for 20 seconds or until butter melts.
3.Stir half the sifted cocoa, sifted flour, caster sugar and chocolate into butter mixture. Sift remaining cocoa and brown sugar over mixture, pour the boiling water gently over mixture.
4.Bake about 40 minutes or until centre is firm. Stand 5 minutes before serving.