Preheat oven to 350°F. Spray a 13x9 non-stick baking pan with cooking spray. Combine boiling water and cocoa in a large bowl. Mix until completely dissolved. Add flour, egg whites, brown and white sugar, baking powder, oil and vanilla; mix well. Mix in chopped walnuts. Pour mix into greased pan. Bake 25 minutes. Cool in pan on wire rack.