1. Combine marshmallows, peanut butter, and butter in a big microwave-safe bowl. Microwave at HIGH 2 minutes. Remove from the microwave using oven mitts. Stir peanut butter mixture vigorously with a wooden spoon until smooth. Add cereal, and stir until well coated. Quickly spoon mixture into a lightly greased 15- x 10-inch jelly-roll pan, and press into an even layer. Cool 5 minutes or until firm.
2. Cut cereal mixture into circles with a 3-inch round cookie cutter. Store excess cereal mixture in an airtight container for snacking on later.
3. Place milk chocolate morsels in a small microwave-safe glass bowl or measuring cup. Microwave at MEDIUM (50% power) 2 minutes or until chocolate looks like it's just beginning to melt. Stir chocolate until smooth.
4. Place white chocolate morsels in a zip-top freezer bag. Partially seal bag, and set upright in a small microwave-safe measuring cup. Microwave at MEDIUM (50% power) 1 minute. Gently squeeze bag until smooth. Snip a tiny hole in 1 corner of bag to create a small opening.
5. Spread melted milk chocolate over top of 1 circle. While milk chocolate is still soft, drizzle white chocolate in 3 rings on top of milk chocolate. Starting at the center, pull a wooden pick through the rings to create a "web." Do the same thing all over again with the remaining circles and chocolate. Refrigerate 30 minutes or until chocolate is firm. Store in an airtight container up to 3 days.