My mom makes these every week--sometimes twice a week for the concession stand at ballgames at our school. One of the high school students re-named these "Grandma Shirley Bars" a couple of years ago and people have been known to cut in line just to get one of them!
Combine cake mix, melted butter and egg. Press into 10x15 pan. Bake at 350 for 10 minutes. Place marshmallows on top. Return to oven for 3 minutes. Melt peanut butter chips, syrup and butter. Add vanilla and pour over peanuts and Krispies. Spread over marshmallows. Let cool. Cut into bars. Note: For the concession stand mom cuts these into 15 bars and individually wraps them once they are cool.