In a food processor or blender, crush the cereal to a fine texture.
Mix peanut butter and margarine. Mix the crushed cereal into the mixture. Mix in the powdered sugar about 1/2 cup at a time. Mixture may look a bit crumbly.
Press mixture firmly into ungreased 13x9x2 pan.
Melt shortening and chips together in a small microwave safe bowl for 30 seconds. Stir and microwave 15 seconds more if needed until all chips are melted and smooth.
Spread chocolate over peanut butter mixture. Chill until chocolate sets, about 1/2 hour. Remove the pan from refrigerator and let the pan sit for about 10 minutes before cutting into squares. Store in airtight container (if there are any left to store).