1Lightly spray 15x10x1-inch pan with cooking spray. In large microwavable bowl, microwave marshmallows, peanut butter and butter uncovered on High 2 minutes. Stir vigorously with wooden spoon until smooth. Add cereal; stir until well coated. Quickly spoon mixture into pan and press evenly. Cool until firm. Cut into rounds with 3-inch round cookie cutter.
2In medium microwavable bowl, microwave chocolate chips uncovered on Medium (50%) 2 minutes, stirring once, until softened and chips can be stirred smooth; set aside. In small resealable freezer plastic bag, place white chips; seal bag. Microwave on Medium (50%) 1 minute or until softened. Gently squeeze bag until chips are smooth. Cut off tiny corner of bag.
3Spread top of 1 round with melted milk chocolate. Immediately drizzle melted white chips in 3 rings on top of milk chocolate. Starting at center, pull toothpick through rings to look like web. Repeat with remaining rounds. Refrigerate 30 minutes or until chocolate is firm.