01 - Place the finely chopped white chocolate in a heatproof bowl.
02 - Gently heat the eggnog in a small saucepan over medium-low until steaming but not boiling.
03 - Pour the hot eggnog over the white chocolate and let stand for 2 minutes, then whisk until smooth and fully melted.
04 - Stir in the softened unsalted butter, ground nutmeg, ground cinnamon, vanilla extract, and salt until well combined.
05 - Cover the bowl and refrigerate the ganache for at least 2 hours, or until firm enough to scoop.
06 - Scoop the chilled ganache using a teaspoon or melon baller and roll into 1-inch balls. Place on a parchment-lined baking tray. Refrigerate for 30 minutes to set.
07 - Melt the chopped dark chocolate in a heatproof bowl set over barely simmering water or in the microwave in short bursts, stirring until smooth.
08 - Using a fork, dip each ganache ball in the melted dark chocolate, allowing excess to drip off, then place them back on the parchment-lined tray.
09 - Immediately dust the tops with a little extra nutmeg or cinnamon, if desired, before the chocolate sets.
10 - Let truffles set completely at room temperature or briefly refrigerate to firm the chocolate. Store in an airtight container in the refrigerator.