1.Preheat the oven to 180°C (160°C fan-forced). Grease a 1.5 litre (6-cup) ovenproof dish and place on an oven tray lined with baking paper or foil (this will catch any drips).
2.Sift the flour and cocoa powder into a medium bowl. Add the sugar and stir to break up any lumps.
3.Whisk together the milk, egg and butter in a jug. Add the milk mixture to the dry ingredients; use a whisk to stir to a smooth batter. Pour into the prepared dish and smooth the surface with the back of a spoon.
4.To make sauce: sift the cocoa powder into a bowl; add the sugar and stir to combine. Sprinkle evenly over the surface of the batter.
5.Pour the boiling water evenly over the cocoa and sugar mixture in the dish. Bake for 50 minutes or until the cake topping is firm. Serve with cream or ice-cream.
We used a 1.5-litre (6-cup) capacity round ovenproof dish, 17cm across and 8cm deep. If a wider, shallower dish is used, the cooking time should be reduced by five to 10 minutes.