In a large microwave-safe bowl, melt butter on high heat for about 45 seconds. Add marshmallows; stir to coat. Microwave on high for 45 seconds; stir. Microwave for 45 seconds longer or until smooth. Stir in peanut butter. Immediatly add cereal; stir gently until coated. Press into greased 13x9x2in baking pan.
In a small microwave safe bowl, heat chocolate chips and shortening on 70% power for 1 minute. Heat in 10 to 20 second intervals until melted; stir until smooth. Spread over cereal mixture. Cool completely.