Preheat oven to 350 F. In a medium bowl, whisk together flour, granulated sugar, 3 tablespoons cocoa powder and salt. Whisk in melted butter, milk and vanilla extract until smooth. Pour the batter into a greased 8 inch baking dish.
In another small bowl, combine brown sugar and the remaining 4 tablespoons cocoa powder. Sprinkle the sugar cocoa powder mixture over the batter without stirring. Pour the boiling water over the batter without stirring. Bake for 45-50 minutes.