In a large mixing bowl, beat together butter and sugar until creamy. Stir in flour and nuts until mixture is crumbly. Reserve ¾ cup of crumb mixture for topping. Press remaining crumbs in bottom of a 15 X 10 X 1 inch cookie sheet. Bake at 350 degrees for about 15 minutes or until crust is golden brown. Meanwhile, combine pumpkin or smashed yams, condensed milk, molasses, eggs, pumpkin pie spice and vanilla. Pour mixture over baked crust. Sprinkle with reserved topping. Bake at 350 degrees for 30-35 minutes or until set. Store covered in refrigerator. Freezes well after baking.