01 - Place finely chopped white chocolate in a heatproof bowl.
02 - Warm eggnog in a small saucepan over medium-low heat until steaming but not boiling.
03 - Pour hot eggnog over white chocolate and allow to sit for 2 minutes, then gently whisk until fully melted and smooth.
04 - Add softened butter, ground nutmeg, ground cinnamon, vanilla extract, and pinch of salt. Stir until well combined.
05 - Cover bowl and refrigerate for a minimum of 2 hours, until ganache is sufficiently firm to scoop.
06 - Using a teaspoon or melon baller, scoop chilled ganache and roll into 1-inch balls. Place onto a parchment-lined baking tray. Refrigerate for 30 minutes to set.
07 - Gently melt dark chocolate in a heatproof bowl set over barely simmering water or in the microwave using short bursts, stirring until smooth.
08 - Using a fork, dip each ganache ball in melted dark chocolate, allowing excess chocolate to drip off. Return to parchment-lined tray.
09 - If desired, immediately dust tops of coated truffles with a pinch of ground nutmeg or cinnamon before chocolate hardens.
10 - Allow chocolate shells to set fully at room temperature or chill briefly until firm. Store in an airtight container in the refrigerator.