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.