01 - Place finely chopped white chocolate in a heatproof bowl.
02 - Gently heat eggnog in a small saucepan over medium-low until steaming, without boiling.
03 - Pour hot eggnog over white chocolate and let stand for 2 minutes. Whisk gently until smooth and fully melted.
04 - Stir in softened butter, nutmeg, cinnamon, vanilla extract, and a pinch of salt until evenly combined. Cover and refrigerate for at least 2 hours until firm.
05 - Scoop chilled ganache using a teaspoon or melon baller. Roll into 1-inch balls and place on a parchment-lined tray. Refrigerate for 30 minutes to set.
06 - Melt dark chocolate in a heatproof bowl set over simmering water (double boiler) or in short microwave bursts, stirring until smooth.
07 - Dip each ganache ball in melted dark chocolate using a fork to coat. Allow excess chocolate to drip off, then return truffles to the tray.
08 - Optionally dust each truffle with an extra pinch of nutmeg or cinnamon before the coating hardens. Let truffles set at room temperature or chill briefly until chocolate is firm.