Bake Betty Crocker devils food cake mix in oblong pan, 13x9x2", as directed on package. Spread Butterscotch Filling over top of cooled cake to within 1/2" of edge. Sprinkle 1/2 cup chopped nuts over filling.
Frost sides and top of cake with Marshmallow Frosting. Melt 1/2 square unsweetened chocolate; dip back of spoon into chocolate to form swirls on frosting.