01 - Place finely chopped white chocolate in a heatproof mixing bowl.
02 - In a small saucepan, heat eggnog over medium-low until steaming but not boiling.
03 - Pour warmed eggnog over white chocolate. Let stand for 2 minutes, then gently whisk until completely smooth and melted.
04 - Add softened butter, ground nutmeg, ground cinnamon, vanilla extract, and salt to the ganache. Stir until fully blended.
05 - Cover the bowl and refrigerate the ganache for at least 2 hours or until firm enough to scoop.
06 - With a teaspoon or melon baller, scoop chilled ganache and roll into 1-inch balls. Arrange on a parchment-lined tray. Refrigerate for 30 minutes to set.
07 - Melt chopped dark chocolate in a heatproof bowl set over a saucepan of barely simmering water or microwave in short bursts, stirring until smooth.
08 - Using a fork, dip each ganache ball into melted dark chocolate, allowing excess to drip off. Place on the parchment-lined tray.
09 - Before the coating sets, garnish with a pinch of ground nutmeg or cinnamon if desired.
10 - Allow truffles to set at room temperature or chill briefly to firm the chocolate shell.