Spray 13x9-inch (3-quart) glass baking dish or pan with cooking spray. Line cookie sheet with waxed paper; set aside.
In 3-quart saucepan, melt butter over medium heat.
Add marshmallows; cook, stirring constantly, until marshmallows are completely melted.
Remove from heat.
Pour mixture into large bowl.
Add cereal; stir with rubber spatula until cereal is thoroughly coated.
Pour cereal mixture into baking dish; press in dish with rubber spatula. Cool completely on cooling rack, about 30 minutes.
Carefully invert baking dish onto waxed paper-lined cookie sheet; remove dish. Using serrated knife, cut cereal mixture into 6 rows by 4 rows to make 24 bars.
In small microwavable bowl, microwave dark chocolate chips on High in 30-second increments, stirring in between, until chips are melted and can be stirred smooth.
Dip one end of each bar into melted chocolate; return to paper-lined cookie sheet to set. Refrigerate 10 minutes or until chocolate is completely set before serving.
