01 - Place the chopped white chocolate in a heatproof bowl.
02 - Heat eggnog in a small saucepan over medium-low until steaming, without boiling.
03 - Pour hot eggnog over white chocolate and let sit for 2 minutes, then whisk gently until smooth and melted.
04 - Stir in butter, nutmeg, cinnamon, vanilla extract, and salt until evenly combined.
05 - Cover the bowl and refrigerate for at least 2 hours, or until the mixture is firm enough to scoop.
06 - Using a teaspoon or melon baller, scoop chilled ganache and roll into 1-inch balls. Place on a parchment-lined tray and refrigerate for 30 minutes.
07 - Melt dark chocolate in a heatproof bowl set over a saucepan of barely simmering water or in short intervals in the microwave, stirring until smooth.
08 - Dip each truffle ball into melted dark chocolate with a fork, allowing excess chocolate to drip back into the bowl. Return truffles to parchment-lined tray.
09 - Dust tops lightly with extra nutmeg or cinnamon before the chocolate sets, if desired.
10 - Let truffles set completely at room temperature or refrigerate briefly until the chocolate coating is firm.