Line an 8 or 9-inch square pan with foil or parchment paper and let it hang over the sides. Butter the foil/paper.
In a heavy, 3-quart saucepan combine the sugar, eggnog and butter. Bring to a rolling boil, stirring constantly. Continue boiling 8 to 10 minutes over medium heat or until a candy thermometer reaches 234°F, stirring constantly to prevent scorching. Once it reaches temperature, it might be thick enough to add the chocolate. But you may need continue to stir and cook for an additional 3 to 5 minutes. You want the texture to get thick like molasses. Remove from heat.
Using a wooden spoon, work very quickly to stir in the chopped white chocolate and nutmeg until the chocolate is melted and smooth. Stir in the marshmallow creme and rum extract. Beat until well blended and then pour into the prepared pan. Sprinkle a little more nutmeg on top. Let stand at room temperature until cooled. Refrigerate if you'd like to speed up the process.
When completely cool, cut into squares. Store in a covered container.
Notes
*This fudge freezes well. Place in a covered container; it will keep for several weeks.