01 - Place finely chopped white chocolate in a heatproof mixing bowl.
02 - Warm eggnog in a small saucepan over medium-low heat until steaming; avoid boiling.
03 - Pour hot eggnog over white chocolate. Let stand for 2 minutes, then gently whisk until smooth and fully melted.
04 - Stir in softened unsalted butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until fully combined.
05 - Cover the bowl and refrigerate the ganache for at least 2 hours, until mixture is firm.
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 30 minutes to set.
07 - In a clean heatproof bowl, melt dark chocolate over a pan of barely simmering water or in the microwave in short intervals, stirring until smooth.
08 - Using a fork, dip each firm ganache ball in melted dark chocolate, allowing excess to drip off. Place coated truffles back on parchment-lined tray.
09 - If desired, dust the tops with a little nutmeg or cinnamon before the chocolate sets.
10 - Allow truffles to set completely at room temperature or briefly chill to firm the chocolate shell.