Preheat oven to 180°C or 160C fan-forced. Grease a 1.5L capacity ovenproof baking dish.
Combine milk, butter and egg and lightly whisk.
Sift flour and cocoa into a large bowl. Stir in sugar. Slowly add milk mixture to flour mixture, whisking until well combined and smooth.
Combine cocoa powder and brown sugar for sauce in a bowl.
Spoon pudding mix into baking dish. Smooth top. Sprinkle combined sugar and cocoa over pudding.
Slowly pour boiling water over the back of a large metal spoon to cover pudding. Place dish onto a baking tray. Bake for 35 to 40 minutes or until pudding bounces back when pressed gently in centre.
Sift cocoa powder over pudding and serve hot with ice-cream.