Grease an 8x8x2-inch baking pan. Set aside.
In a medium bowl stir together the flour, granulated sugar, the 2 tablespoons cocoa powder, the baking powder, and salt. Stir in the milk, oil, and vanilla. Stir in the walnuts. Pour into the prepared pan.
In another medium bowl stir together the brown sugar and the 1/4 cup cocoa powder. Stir in the boiling water; slowly pour over batter in pan.
Bake in a 350 degree F oven for 40 minutes. Serve warm. If desired, serve with ice cream. Makes 6 to 8 servings.