Combine cake mix and margarine in bowl and mix well. Reserve 1 cup of cake mixture. Press remaining mixture in greased 9×3-inch baking pan. Combine peanut butter and marshmallow cream in bowl and mix well. Spread peanut butter mixture evenly over cake mix layer. Sprinkle reserved cake mixture over top. Bake at 350° 20 minutes. Let stand until cool. Cut into bars. Makes 12 to 16.