Line 13x9-inch pan with foil. Spray with cooking spray.
In 4-quart microwavable bowl, microwave butter uncovered on High about 1 minute or until melted.
Add marshmallows; toss to coat.
Microwave about 2 minutes, stirring after each minute, until marshmallows are completely melted and mixture is well blended.
Add cereal; mix well.
Spoon cereal mixture into pan.
Spray hands with cooking spray, and press mixture firmly in pan.
Cover with waxed paper; press second 13x9-inch pan on top to compress bars.
Remove pan and paper. Cool completely, about 1 hour.
Remove bars from pan; peel off foil. Using sharp knife, cut bars into 8 rows by 6 rows to make 48 squares.
Line cookie sheet with waxed paper.
In 1-quart microwavable bowl, microwave 12 oz of the chocolate and 1 tablespoon of the shortening uncovered on High 1 minute 30 seconds; stir.
Continue microwaving and stirring in 15-second intervals until melted and smooth.
Using 2 forks, dip and roll each cereal square in coating.
Place on cookie sheet.
Repeat twice to use up chocolate, shortening and squares.
Sprinkle coated squares with black sugar and/or sprinkles.
Place in freezer 15 minutes or refrigerator 30 minutes until chocolate is set.
Store in airtight container or sealed storage bag.