01 - Place chopped white chocolate in a heatproof bowl.
02 - Heat eggnog in a small saucepan over medium-low until steaming, without boiling.
03 - Pour hot eggnog over white chocolate and let stand for 2 minutes, then whisk until completely melted and smooth.
04 - Stir in unsalted butter, nutmeg, cinnamon, vanilla extract, and salt until fully combined.
05 - Cover and refrigerate ganache for at least 2 hours until firm enough to scoop.
06 - Scoop chilled ganache into 1-inch balls using a teaspoon or melon baller. Arrange on a parchment-lined baking tray and refrigerate for 30 minutes.
07 - Melt dark chocolate in a heatproof bowl set over a pan of barely simmering water or microwave in short intervals, stirring until smooth.
08 - Dip each ganache ball into melted dark chocolate using a fork, letting excess chocolate drip off, then return to parchment-lined tray.
09 - Dust truffles lightly with nutmeg or cinnamon before the chocolate sets, if desired.
10 - Allow truffles to set fully at room temperature or chill briefly to firm up the chocolate shell.