Line a 15x10x1-in. pan with foil; grease foil with 1 tablespoon butter., In a heavy large saucepan, combine sugars, milk and cubed butter. Bring to a rolling boil over medium heat, stirring constantly; cook and stir 5 minutes. Remove from heat. Stir in marshmallows and vanilla until blended., Gradually stir in chocolate chips and chopped chocolate until melted. Fold in pecans. Immediately spread into prepared pan. Refrigerate 1 hour or until firm. Using foil, lift fudge out of pan. Remove foil; cut fudge into 1-in. squares. Store between layers of waxed paper in an airtight container.