Line an 8” x 8” baking dish with parchment paper and spray lightly with food cooking spray.
In a heavy saucepan, bring butter, eggnog and sugar to a boil then reduce heat to medium-high and continue to boil (stirring constantly because this could easily burn with all the sugar) until candy thermometer temperature reaches 234F. This takes about 10 minutes of stirring and boiling.
Remove from heat then immediately stir in white chocolate and stir until melted.
Stir in marshmallow crème until completely combined.
Stir in vanilla and ½ teaspoon nutmeg (leave remaining ½ teaspoon nutmeg for topping).
Spread evenly in prepared baking dish. Sprinkled remaining ½ teaspoon nutmeg over top.
Chill in fridge at least 4 hours before cutting.
Storage: Store leftovers in fridge in a sealed food storage container for 4-5 days.