In a large bowl, combine first six ingredients. Add oil, eggs, buttermilk and vanilla; mix well. Fold in prunes and nuts. Pour into an ungreased 13-in. x 9-in. baking pan. Bake at 350° for 45 minutes. meanwhile, combine all topping ingredients in a saucepan. bring to a boil and boil 2 minutes. Pour over hot cake. Leave in pan to cool. Yield: 12-16 servings.