Preheat oven to 170C / 150C fan forced.
Grease a 5 - 6 cup baking dish with butter.
Whisk brown sugar and cocoa in a bowl, set aside.
Whisk flour, baking powder, caster sugar, cocoa powder and salt in a bowl.
.In a separate bowl or jug, whisk together the butter and milk, then whisk in the egg and vanilla.
Pour the egg mixture into the flour mixture. Mix until combined - it will be a thick batter.Spread into baking dish.
Sprinkle with sugar / cocoa mixture. Shake gently to spread out thinly.Carefully pour the hot water over the back of a dessert spoon all over the top of the pudding
Transfer to oven and bake for 30 minutes, or until the top of the cake springs back when poked lightly. The top will be a bit crusty, like the top of brownies.Remove from oven, then serve immediately. Top with cream or icecream.