1Heat oven to 350ºF (325ºF for dark or nonstick pan). Grease bottom only of 9-inch square pan with shortening or cooking spray.
2In large bowl, beat cake mix, oil, peanut butter and eggs with electric mixer on low speed until smooth. Spread in pan.
3Bake 40 to 45 minutes or until toothpick inserted halfway between edge of pan and center comes out with just a few moist crumbs attached. Cool completely on cooling rack, about 1 hour.
4In medium bowl, beat butter and marshmallow creme with electric mixer on medium speed. Add vanilla. Gradually beat in powdered sugar. Thin with milk if necessary. Spread frosting on cooled bars. Cut into 4 rows by 4 rows.