Preheat oven to 350 degrees F. In a bowl, sift flour, sugar, 2 tablespoons cocoa, baking powder and salt together. Add the milk, nuts, vanilla and butter and mix until smooth. Pour into a 9 by 13-inch baking pan. Mix the brown sugar, water and 4 tablespoons cocoa well, and pour over the batter. Bake for about 45 minutes.