In a medium bowl stir together the 2/3 cup sugar, flour, 5 tablespoons of the cocoa, baking powder and salt. Blend in milk, melted butter and vanilla; beat until smooth. Pour batter into one 20cm square baking dish.
In separate bowl stir together the remaining white sugar, brown sugar and the remaining cocoa; sprinkle mixture evenly over batter. Pour hot water over top. Do not stir!
Bake for 35 to 40 minutes or until centre is almost set. Let stand for 15 minutes. Then ladle into dessert bowls spooning sauce from bottom of pan over top.