Microwave butter in large microwaveable bowl on HIGH 45 sec. or until melted.
Add marshmallows; toss to evenly coat. Microwave 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 foil-lined 13x9-inch pan sprayed with cooking spray.
Microwave caramels and milk in medium microwaveable bowl 2 to 2-1/2 min. or until caramels are completely melted, stirring after each minute; pour over cereal mixture.
Spread to completely cover cereal mixture.
Sprinkle with nuts; press lightly intocaramel with back of spoon to secure.
Place chocolates in small microwaveable bowl. Microwave 1 min. or until chocolates are completely melted, stirring every 30 sec.; drizzle over nuts. Cool before cutting to serve.
