Line a 13x9x2-inch baking pan with foil, extending foil over edges of pan. Butter foil; set aside.
Butter sides of a heavy 3-quart saucepan. In saucepan, combine sugar, evaporated milk and salt. Cook and stir over medium-high heat until mixture boils. Reduce heat to medium; continue cooking and stirring for 10 minutes.
Remove pan from heat. Add chocolate bar; chocolate pieces; marshmallow crème; if you like, pecans and the vanilla. Stir until chocolate melts and mixture is combined. Beat by hand for 3 to 5 minutes or until mixture starts to become thicker.
Immediately pour fudge in the prepared pan; shake pan gently to spread fudge to edges of pan. Cover; chill for 2 to 3 hours or until firm. When fudge is firm, use foil to lift it out of pan. Cut fudge. Store, tightly covered. Refrigerate, if necessary, if fudge seems soft.