Preheat oven to 350 degrees. In a large bowl, sift together flour, 1 cup granulated sugar, 3 tablespoons cocoa, baking powder, and salt. Beat in milk, butter, and vanilla. Stir in chopped walnuts. Pour into a greased 9x13-inch pan. Mix together brown sugar, remaining granulated sugar, and remaining cocoa. Sprinkle over the cake batter. Pour boiling water over the top. Bake for 40 minutes.