01 - Place chopped white chocolate into a heatproof bowl.
02 - Gently heat eggnog in a small saucepan over medium-low until steaming, without boiling.
03 - Pour hot eggnog over white chocolate. Allow to sit for 2 minutes, then gently whisk until smooth and fully melted.
04 - Stir in unsalted butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until well combined.
05 - Cover the bowl and refrigerate for at least 2 hours, or until the ganache is firm enough to scoop.
06 - Using a teaspoon or melon baller, scoop chilled ganache and roll into 1-inch balls. Arrange on a parchment-lined baking tray and refrigerate for 30 minutes to set.
07 - Melt dark chocolate in a heatproof bowl over barely simmering water (double boiler) or microwave in short intervals, stirring until smooth.
08 - Using a fork, dip each ganache ball in the melted dark chocolate, allowing excess to drip off. Return to the parchment-lined tray.
09 - Optionally, dust the tops with ground nutmeg or cinnamon while chocolate is still soft. Let the truffles set at room temperature or briefly chill to firm the coating.