Microwave 2 cups marshmallows in large microwaveable bowl on HIGH 1 min. or until marshmallows are completely melted when stirred.
Add cream cheese; beat with mixer until blended.
Add dry pudding mix; mix well. Gradually add milk, mixing well after each addition.
Whisk in 2-1/2 cups COOL WHIP and 1/2 cup chopped chocolate.
Spoon dip into serving bowl. Add remaining COOL WHIP; swirl gently with spoon to marbleize.
Top with remaining chocolate and marshmallows before serving.