Cut a piece of parchment paper to line a shallow 9 x 13 inch baking sheet. Let paper hang over the sides about 3 inches for easy fudge removal. (An 8-inch baking pan can also be used for larger squares of fudge) In a heavy 3-quart saucepan, melt butter, sugar, eggnog and salt and cook over medium heat until well combined. Bring mixture to a boil and cook until candy thermometer reads 234degrees (soft ball stage). Stir occasionally so mixture doesn't scorch, especially at the end of the cooking process. When mixture reaches 234, stir in white chocolate until dissolved. Stir in marshmallow creme and vanilla and stir well to combine. Pour into prepared pan. If desired, lightly and very sparingly sprinkle ground nutmeg and decorative sugar over fudge. Let stand at room temperature until fudge has cooled. Cover and refrigerate until firm (at least 2 hours or overnight). Cut into desired squares. Eggnog fudge can be stored in the refrigerator or frozen in a well-sealed container for 2-3 weeks.
Leftover Eggnog Fudge Version 2:
Line an 8-inch square pan with parchment paper. Set aside. In a medium saucepan, combine sugar, eggnog, corn syrup and butter. Cook over medium heat until sugar dissolves and mixture comes to a boil. Insert cookie thermometer and cook until mixture reaches 234 degrees or soft ball stage. Pour mixture into a stainless or glass mixing bowl. Cool to lukewarm, or 110 degrees. Add vanilla and beat with electric mixer just until thickened. (If you mix too much beyond this stage, the mixture will seize up and crystallize.) Spread fudge into prepared pan. Refrigerate until firm. Cut into squares and store in refrigerator. Makes about 1 pound of fudge.