01 - Place the finely chopped white chocolate in a heatproof bowl.
02 - Gently heat the eggnog in a small saucepan over medium-low until steaming but not boiling.
03 - Pour the hot eggnog over the white chocolate. Let sit for 2 minutes, then whisk until completely smooth.
04 - Stir in softened butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until well incorporated.
05 - Cover the bowl and refrigerate for at least 2 hours until the mixture is firm enough to scoop.
06 - Using a teaspoon or melon baller, scoop firm ganache and roll into 1-inch balls. Arrange on a parchment-lined baking tray and refrigerate for 30 minutes.
07 - Melt the dark chocolate in a heatproof bowl set over a pan of barely simmering water or in the microwave with short bursts, stirring until smooth.
08 - Using a fork, dip each ganache ball into the melted dark chocolate and let excess chocolate drip off. Place back on the parchment-lined tray.
09 - Optionally, sprinkle truffles with a pinch of nutmeg or cinnamon before the coating sets. Allow truffles to set at room temperature or refrigerate briefly.