Preheat oven to350°F (175°C). Lightly grease a 9-inch square baking pan, or an 8 cup / 2 quart size baking dish. Pre-boil the water for the sauce.
FOR THE CAKE:
Combine flour, baking powder, salt, 1/2 cup sugar, and 1/4 cup cocoa.In a separate bowl, combine the melted butter, milk and vanilla. Add to the dry ingredients, mixing until smooth and lump free.
Pour the batter into prepared baking pan or dish and spread evenly.
FOR THE SAUCE:
Combine the brown sugar, and remaining 4 tablespoons of cocoa. Sprinkle evenly overtop of the cake batter.Pour the boiling water over the batter, but do not mix.Note: allow the liquid to sit on top of the batter, it does not need to be mixed in. As the pudding cake bakes the liquid will form the fudge sauce.
Bake 35 minutes or until the centre is almost set. Remove from oven. Allow to cool for 15 minutes.
Serve a scoop of the cake with some of the fudge sauce spooned over top. Delicious on it's own, or with whipped cream or vanilla ice cream.