Line an 8-inch or 9-inch square pan with parchment paper or foil, leaving an overhang on two sides. Lightly grease the lining so the chilled fudge releases easily.

Add the sugar, butter, prepared eggnog, cinnamon, nutmeg, and optional salt to a heavy saucepan. Heat over medium heat, stirring constantly, until the butter melts and the mixture reaches a full rolling boil.

Continue boiling and stirring constantly until the mixture reaches 234 to 238°F on a candy thermometer, which is the soft-ball stage. This usually takes about 5 to 7 minutes after the boil begins.

Remove the saucepan from the heat immediately. Quickly stir in the white chocolate chips, marshmallow creme, vanilla extract, and rum extract until the mixture is smooth and fully incorporated.

Pour the hot fudge mixture into the prepared pan and smooth the surface. Let it cool at room temperature, then refrigerate for 2 to 4 hours, or until firm.

Lift the fudge from the pan using the parchment or foil overhang and remove the lining. Cut into small squares with a large knife warmed under hot water and dried between cuts.