Microwave first 3 ingredients in large microwaveable bowl on HIGH 1-1/2 min. or until marshmallows are completely melted and mixture is well blended, stirring after 45 sec.
Add cereal; mix well.
Press onto bottom of 13x9-inch pan sprayed with cooking spray.
Drizzle with melted chocolate.
Decorate with chocolate pieces, pressing gently into cereal mixture to secure. Cool completely.