01 - Place finely chopped white chocolate in a heatproof bowl.
02 - Gently warm eggnog in a small saucepan over medium-low heat until steaming but not boiling.
03 - Pour hot eggnog over white chocolate and let stand for 2 minutes. Whisk gently until the chocolate is completely melted and smooth.
04 - Stir in butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until fully 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 the chilled ganache and roll into 1-inch balls. Arrange them on a tray lined with parchment paper and chill for 30 minutes.
07 - Melt dark chocolate in a heatproof bowl set over a saucepan with barely simmering water, or microwave in short bursts, stirring until smooth.
08 - With a fork, dip each ganache ball into the melted dark chocolate, allowing excess to drip off. Place truffles back onto the parchment-lined tray.
09 - Optionally, dust each truffle with a pinch of ground nutmeg or cinnamon before the chocolate sets. Allow truffles to set at room temperature or refrigerate briefly until the coating is firm.