Preheat oven to 170°C (335°F) conventional heating or 150°C fan-forced. Grease a 5-6 cup capacity baking dish and set aside.Topping: Mix brown sugar and cocoa, in a small bowl until combined.In another bowl, whisk together flour, baking powder, caster sugar, cocoa powder and a pinch of salt.
In another mixing bowl or pitcher, whisk together melted butter, adding in milk, then egg, and vanilla extract, till well incorporated.
Pour the wet ingredients into the dry and mix until thick batter is achieved. Spread batter evenly into prepared dish. Sprinkle sugar and cocoa topping over batter, gently shaking dish to distribute evenly.
Carefully pour hot water over the back of a dessert spoon and let it flow gently over the pudding. Refer to reference photo or video in recipe for technique.
Put the dish in the oven and bake until about 30 minutes, or until the top of pudding springs back when lightly touched. The surface will have a somewhat crispy texture, much like a brownie.
Once done, take it out of the oven and let it rest for just a couple of minutes before serving.