This s'more-ish mixture is very sticky, so try coating your hands with cooking spray before pressing it into the pan.
Combine the peanut butter and butter in a large microwave-safe bowl. Microwave at high 45 seconds or until mixture melts. Add marshmallows; microwave at high 1 1/2 minutes or until smooth, stirring every 30 seconds. Add cereal to peanut butter mixture; toss until well-combined. Pat cereal mixture into a 13 x 9-inch baking pan coated with cooking spray.
Place peanut butter chips in a small microwave-safe bowl. Microwave at high 30 seconds or until chips melt. Spoon melted chips in a small heavy-duty zip-top plastic bag; seal. Snip a tiny hole in 1 corner of bag; drizzle melted chips over cereal mixture. Cool slightly; cut into 24 pieces.