Line 8- or 9-inch square pan with foil.

Butter foil; set aside.

Combine sugar, marshmallow creme, evaporated milk and butter in heavy medium saucepan.

Cook over medium heat, stirring constantly, to a full boil.

Boil, stirring constantly, 5 minutes.

Remove from heat; add chocolate chips and vanilla.

Stir just until chips are melted; pour into prepared pan.

Refrigerate 1 hour or until firm.

Cut into squares.

Store tightly covered in a cool, dry place.