01 - Place the finely chopped white chocolate into a heatproof bowl.
02 - Gently warm the eggnog in a small saucepan over medium-low heat until steaming, without allowing it to boil.
03 - Pour the hot eggnog over the white chocolate. Let it sit for 2 minutes, then whisk gently until smooth and fully melted.
04 - Stir in the softened butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until well combined.
05 - Cover the bowl and refrigerate for at least 2 hours, or until the mixture is firm enough to scoop.
06 - Scoop the chilled ganache using a teaspoon or melon baller and roll into 1-inch balls. Arrange on a parchment-lined baking tray. Refrigerate for 30 minutes to set.
07 - Melt the dark chocolate in a heatproof bowl set over a pan of barely simmering water or microwave in short bursts, stirring until smooth.
08 - Using a fork, dip each ganache ball into the melted dark chocolate, allowing excess to drip off. Place back on the parchment-lined tray.
09 - Optionally, dust the tops of the truffles with a pinch of nutmeg or cinnamon before the chocolate sets.
10 - Allow truffles to set completely at room temperature or briefly chill them to firm. Store in an airtight container in the refrigerator for up to 1 week.