Line a 9-in.-square baking pan with foil; grease foil with 1 teaspoon butter. In a large heavy saucepan, combine sugar, milk and remaining butter. Bring to a rapid boil over medium heat, stirring constantly. Cook and stir 4 minutes.
Remove from heat. Stir in baking chips and marshmallow creme until melted. Pour one-third of the mixture into a small bowl; stir in vanilla.
To remaining mixture, stir in root beer concentrate; immediately spread into prepared pan. Spread vanilla mixture over top. 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 in the refrigerator.Yield: about 3 pounds.