In a large bowl, combine flour, sugar, 1/4 cup cocoa, baking powder and salt. Blend in milk, oil and vanilla. Stir in nuts. Pour into a greased square baking pan. Combine brown sugar and remaining cocoa; sprinkle evenly over batter. Pour hot water over top. Do not stir. , Bake at 350° for 45 minutes or until cake tests done. Cool 10 minutes. Cut into squares and invert onto plates. Serve with ice cream or whipped cream. If cake is not served immediately, pieces can also be warmed in the microwave.