Line a 9 inch square pan with parchment paper and set aside.
In a large saucepan, combine sugar, butter, and eggnog. Bring to a boil while stirring constantly, and heat until it reaches soft ball stage 234°F/113°C (about 4 minutes). Remove from heat.
Stir in white chocolate until it’s fully melted and incorporated.
Stir in marshmallow cream, nutmeg, cinnamon, salt, and rum. Keep stirring until the mixture is smooth then spread into the prepared pan (You can use a mixer with whisk attachment to beat until well blended).
Very lightly dust the top of the fudge with grated nutmeg, and let the fudge set at room temperature or in the fridge (4 hours – overnight).
Slice into squares and serve.
Notes
Homemade fudge will stay fresh in the fridge for up to 3 weeks.