01 - Place the finely chopped white chocolate into a heatproof bowl.
02 - Gently heat the eggnog in a small saucepan over medium-low until steaming, without allowing it to boil.
03 - Pour the hot eggnog over the white chocolate and let stand for 2 minutes. Gently whisk until completely smooth and melted.
04 - Stir in the softened butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until evenly combined.
05 - Cover the bowl and refrigerate for at least 2 hours, until the ganache is firm enough to scoop.
06 - Scoop 1-inch portions of the chilled ganache using a teaspoon or melon baller, roll into balls, and arrange on a parchment-lined tray. Refrigerate for 30 minutes to set.
07 - Melt the dark chocolate in a heatproof bowl set over a saucepan of barely simmering water (double boiler) or microwave in short intervals, stirring until smooth.
08 - Use a fork to dip each ganache ball in the melted dark chocolate, allowing excess to drip off. Place back on the parchment-lined tray.
09 - If desired, dust the tops of each truffle with a small pinch of nutmeg or cinnamon before the chocolate sets.
10 - Let truffles set at room temperature or briefly refrigerate to firm the coating before serving or packaging.