01 - Place the finely chopped white chocolate into a heatproof bowl.
02 - Gently heat the eggnog in a small saucepan over medium-low flame until just steaming, ensuring it does not reach a boil.
03 - Pour the warmed eggnog over the white chocolate and allow to stand for 2 minutes. Whisk gently until smooth and melted.
04 - Stir in the unsalted butter, ground nutmeg, ground cinnamon, vanilla extract, and salt until fully combined and glossy.
05 - Cover the bowl and refrigerate for at least 2 hours, or until the mixture is firm enough to scoop and shape.
06 - Using a teaspoon or melon baller, scoop portions of ganache and roll into 1-inch balls. Arrange on a parchment-lined tray and refrigerate for 30 minutes to set.
07 - Melt the chopped dark chocolate in a heatproof bowl set over a pan of barely simmering water (double boiler), or microwave in short bursts stirring until fully smooth.
08 - With a fork, dip each chilled ganache ball into the melted chocolate, allowing excess to drip off. Place coated truffles back onto the lined tray.
09 - Optionally, dust the tops with a small pinch of nutmeg or cinnamon before the chocolate hardens. Let truffles set completely at room temperature or refrigerate briefly for a firmer shell.