Scoop 1/4 cup ice cream into eight serving dishes. Place on chilled baking sheet. Freeze uncovered no longer than 24 hours. Preheat oven to 350. Prepare a 9" x 9" pan with cooking spray and flour; set aside. In a mixing bowl, sift together flour, granulated sugar, two tablespoons cocoa powder, and baking powder. In another mixing bowl, combine milk, oil, vanilla extract, and walnuts. Mix dry ingredients with wet ingredients just until moistened. Spread into prepared pan. Sprinkle with brown sugar and remaining cocoa powder. Pour hot water over batter. Bake for 40 minutes. Let stand 15 minutes. Top ice cream with cake and spoon sauce over each serving.