Microwave chocolate morsels in a large glass bowl at HIGH for 2 minutes or until melted, stirring every 30 seconds. Stir in peanut butter, stirring until well blended. Stir in cereal, peanuts, and marshmallows.
Line a 13x9-inch pan with foil. Lightly grease foil. Press mixture into foil-lined pan. Drizzle with chocolate. Sprinkle with white nonpareils, if desired.
Let stand 1 hour or until firm; cut with a 3-inch heart-shaped cookie cutter to make hearts. Store hearts in an airtight container or place in a candy box, if desired.