Combine granulated sugar, heavy cream, butter, and eggnog in a heavy-bottomed saucepan.
Cook over medium heat, stirring constantly with a silicone spatula, until the mixture reaches the soft-ball stage (235-240°F or 112-115°C on a candy thermometer).
Remove from heat and stir in vanilla extract, ground nutmeg, ground cinnamon, and salt.
If using, stir in white chocolate chips or chopped crystallized ginger.
Pour the fudge into a prepared (lined with parchment paper) 8×8 inch baking pan.
Let the fudge cool completely at room temperature, then chill in the refrigerator for at least 2 hours before cutting into squares.