01 - Place the finely chopped white chocolate into a heatproof bowl.
02 - Gently 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 and let stand for 2 minutes. Whisk gently until melted and smooth.
04 - Mix in softened butter, nutmeg, cinnamon, vanilla extract, and a pinch of salt until fully combined.
05 - Cover and refrigerate the ganache for at least 2 hours, until it is firm enough to scoop.
06 - Scoop out portions of chilled ganache using a teaspoon or melon baller, rolling each into a 1-inch ball. Place on a parchment-lined baking tray.
07 - Return the tray to the refrigerator for 30 minutes to allow the truffles to firm up.
08 - Melt the dark chocolate in a heatproof bowl set over a pan of barely simmering water or in short microwave bursts, stirring until smooth.
09 - Dip each chilled ganache ball in the melted dark chocolate using a fork, allowing any excess to drip off, then place them back on the parchment tray.
10 - Optional: Dust the tops with ground nutmeg or cinnamon before the chocolate sets. Allow truffles to set at room temperature or briefly chill to firm the chocolate shell.