Preheat oven to 350 F / 175 C. Lightly grease a 9 inch baking dish. Also pre boil water in kettle for the chocolate sauce.
For the Cake:
In a medium sized bowl, combine the dry ingredients: flour, white granulated sugar, cocoa powder, baking powder and salt. Whisk lightly until combined. Then add the wet ingredients: milk, butter and vanilla extract. Mix until combined and fairly smooth without too many lumps. Batter will be thick.
Pour the batter into the prepared baking dish and spread evenly.
For the Chocolate Sauce:
Combine the dry ingredients: granulated sugar, brown sugar and cocoa powder in a bowl. Sprinkle this mix evenly onto the surface of the cake batter. Do not mix.
Slowly pour the boiling water onto the surface of this cake batter, making sure it evenly covers the whole surface of the baking dish. DO NOT MIX OR STIR THE WATER INTO THE BATTER.
Place in the oven, and bake for about 35 minutes, until the edges of set and the centre is only slightly jiggly and is almost set.
Remove from oven and wait 10 minutes before serving.