Preheat the oven to 180C/350F. Grease a 9 x 13-inch pan or baking dish and set aside.
In a large mixing bowl, combine the sugar, flour, cocoa powder, baking powder, salt, milk, vanilla, and melted butter, and mix well. Transfer to the greased pan.
In a separate bowl, combine the white sugar, brown sugar, and cocoa powder. Sprinkle the sugar and cocoa mixture over the top of the cake. Gently pour the hot water over the top of the cake. Do not mix or whisk it.
Bake the pudding bake for 35-40 minutes, or until the top is just done. Remove from the oven and let cool for 15 minutes, before serving.
