01 - Place the finely chopped white chocolate in a heatproof bowl.
02 - Warm the eggnog gently in a small saucepan over medium-low heat until steaming, avoiding boiling.
03 - Pour the heated eggnog over the white chocolate and let sit for 2 minutes. Gently whisk until fully melted and smooth.
04 - Stir in softened butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until thoroughly combined.
05 - Cover the bowl and refrigerate for at least 2 hours, or until the ganache is firm enough to scoop.
06 - Using a teaspoon or melon baller, portion and roll the chilled ganache into 1-inch balls. Arrange on a baking tray lined with parchment paper and refrigerate for 30 minutes.
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 between intervals until smooth.
08 - With a fork, dip each ganache ball into the melted dark chocolate, letting excess chocolate drip off. Transfer to the parchment-lined tray.
09 - Optionally, sprinkle each truffle with a light dusting of nutmeg or cinnamon before the chocolate sets.
10 - Let truffles set fully at room temperature or refrigerate briefly to firm the chocolate coating.