Preheat oven to 350. Mix first 6 ingredients until well blended and pour into a 9x13 greased pan. Sprinkle cake mix on top and then cover with walnuts. Pour melted butter over top and bake about 40 minutes, poke center with toothpick to see if it's done, bake additional 10 minutes if needed.
Let cake cool on wire rack and top with cool whip when served.