01 - Place finely chopped white chocolate in a heatproof bowl.
02 - Warm eggnog in a small saucepan over medium-low until steaming but not boiling.
03 - Pour steamed eggnog over the white chocolate. Let stand for 2 minutes, then whisk gently until smooth and fully melted.
04 - Add softened butter, ground nutmeg, ground cinnamon, vanilla extract, and salt. Stir until fully incorporated.
05 - Cover the bowl and refrigerate for at least 2 hours, or until the mixture is firm and scoopable.
06 - Scoop chilled ganache into 1-inch balls using a teaspoon or melon baller. Roll between palms for uniform shape. Arrange on a parchment-lined tray. Refrigerate for 30 minutes to set.
07 - Melt chopped dark chocolate in a heatproof bowl set over barely simmering water (double boiler) or microwave in 20-second increments, stirring until smooth.
08 - Using a fork, dip each ganache ball into melted dark chocolate, allowing excess to drip off. Place on parchment-lined tray.
09 - If desired, lightly dust each truffle with ground nutmeg or cinnamon while chocolate coating is soft.
10 - Allow chocolate shells to set completely at room temperature or refrigerate briefly until firm.