Combine melted butter and dry cake mix (let butter cool down a couple of minutes before adding to dry mix. Mixture should be crumbly. If butter is too hot, the result will be a sticky mass rather than a crumbly mixture.) Reserve 1.5 cups of this mixture for topping. Pat remaining crumb mixture into an ungreased 9 x 13 pan. Combine peanut butter and marshmallow creme and spread evenly over the crumb mixture in the pan. Crumble the reserved 1.5 cups of cake mixture over the peanut butter/marshmallow creme layer. Bake 20 minutes at 180ºC Let cool, cut into bars, and serve one piece at a time. Makes 3 dozen bars.