01 - Place the finely chopped white chocolate into a heatproof bowl.
02 - Warm the eggnog in a small saucepan over medium-low heat until steaming but not boiling.
03 - Pour the hot eggnog over the white chocolate. Let stand for 2 minutes, then whisk gently until completely smooth and melted.
04 - Blend in the unsalted butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until fully incorporated.
05 - Cover the bowl and refrigerate the mixture for at least 2 hours, or until the ganache is firm and scoopable.
06 - Using a teaspoon or melon baller, portion out the chilled ganache and roll into 1-inch (2.5 cm) balls. Arrange on a parchment-lined baking tray and refrigerate for 30 minutes to set.
07 - Melt the chopped dark chocolate in a heatproof bowl set over a saucepan with barely simmering water or in short microwave bursts, stirring frequently until smooth.
08 - Using a fork, dip each ganache ball into the melted dark chocolate, letting excess drip off. Transfer to the parchment-lined tray.
09 - Optionally, dust tops with a pinch of ground nutmeg or cinnamon before the chocolate hardens. Allow truffles to set at room temperature, or briefly refrigerate for a firmer shell.