Line a 9"x13" inch pan with non-stick foil; set aside.
In a large, heavy saucepan, combine sugars, butter, eggnog, marshmallows and salt.
2 cups sugar, 2 cups packed brown sugar, 16 tbsp unsalted butter, 10 oz marshmallows, 1/2 tsp salt, 1 1/4 cup eggnog
Bring to a rolling boil, stirring constantly. Continue boiling and stirring until a candy thermometer reads 240°F.
Remove from the heat. Using an electric mixer immediately beat in white chocolate, vanilla and spices until well blended. Continue beating while the mixture cools slightly and begins to thicken and hold trails and ridges in the surface for a few seconds.
16 oz white chocolate chips, 2 tsp vanilla, 1 tsp ground nutmeg, 1/2 tsp cinnamon, 1/8 tsp allspice
Pour into the prepared pan, smooth the top and sprinkle ground nutmeg over the surface.
1 tsp ground nutmeg
Allow the fudge to cool and set completely before cutting into squares.