In a medium bowl, mix together cake mix and butter. Set aside 1 1/2 cups of the mixture, and press remainder into the bottom of an ungreased 9x13 inch baking pan. In a medium bowl, stir together peanut butter and marshmallow creme; spread over the crust in the pan. Crumble remaining cake mix mixture over top.
Bake 20 minutes in the preheated oven. Cool, and cut into squares to serve.
