01 - Place the finely chopped white chocolate in a heatproof bowl.
02 - Gently warm the eggnog in a small saucepan over medium-low until steaming, without boiling.
03 - Pour hot eggnog over the white chocolate. Let stand for 2 minutes, then gently whisk until fully melted and smooth.
04 - Stir in the softened butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until evenly combined.
05 - Cover the bowl and refrigerate for at least 2 hours, or until the mixture is firm enough to scoop.
06 - With a teaspoon or melon baller, scoop and roll the chilled ganache mixture into 1-inch balls. Transfer them onto a parchment-lined baking tray. Refrigerate for 30 minutes to set.
07 - Melt chopped dark chocolate in a heatproof bowl set over a pan of gently simmering water (double boiler), or microwave in short intervals, stirring until silky and smooth.
08 - Using a fork, dip each ganache ball into the melted dark chocolate, allowing excess to drip off. Place coated truffles back onto the parchment-lined tray.
09 - Optional: While the chocolate is still soft, dust tops with extra nutmeg or cinnamon. Allow truffles to set fully at room temperature, or briefly refrigerate for a quicker finish.