01 - Place the chopped white chocolate in a heatproof bowl.
02 - Gently warm the eggnog in a small saucepan over medium-low heat until steaming without bringing it to a boil.
03 - Pour the hot eggnog over the white chocolate and let stand for 2 minutes. Whisk gently until completely smooth and melted.
04 - Incorporate the softened butter, ground nutmeg, ground cinnamon, vanilla extract, and salt into the ganache mixture, mixing until thoroughly combined.
05 - Cover and refrigerate the ganache for at least 2 hours until firm enough for shaping.
06 - Using a teaspoon or melon baller, scoop and roll the chilled ganache into 1-inch balls. Arrange on a parchment-lined tray and refrigerate for 30 minutes to set.
07 - Melt the dark chocolate in a heatproof bowl set over barely simmering water or in short microwave intervals, stirring until smooth.
08 - Using a fork, dip each ganache ball into the melted dark chocolate, allowing excess to drip off. Return to the parchment-lined tray.
09 - If desired, dust the tops lightly with ground nutmeg or cinnamon before the chocolate sets.
10 - Allow truffles to set fully at room temperature or quickly chill to firm the coating before storing.