01 - Place finely chopped white chocolate in a heatproof bowl.
02 - Warm the eggnog in a small saucepan over medium-low heat until steaming but not boiling.
03 - Pour heated eggnog over the white chocolate. Allow to stand for 2 minutes, then whisk gently until smooth and fully melted.
04 - Stir in softened unsalted butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until thoroughly combined.
05 - Cover bowl tightly and refrigerate for a minimum of 2 hours until firm enough to scoop.
06 - Using a teaspoon or melon baller, portion and roll the chilled ganache into 1-inch balls. Arrange on a tray lined with parchment paper and refrigerate for 30 minutes to firm.
07 - Melt dark chocolate in a heatproof bowl set over barely simmering water, or microwave in short bursts, stirring until smooth and fluid.
08 - With a fork, dip each chilled ganache ball into melted dark chocolate, allow excess chocolate to drip off, and return to the parchment-lined tray.
09 - If desired, dust truffle tops with a small pinch of ground nutmeg or cinnamon before the chocolate coating sets.
10 - Let truffles set at room temperature or briefly chill until chocolate shell is firm.