01 - Place the finely chopped white chocolate in a heatproof bowl.
02 - Warm the eggnog gently in a small saucepan over medium-low heat until just steaming. Do not bring to a boil.
03 - Pour the hot eggnog over the white chocolate and let stand for 2 minutes. Whisk gently until the chocolate is fully melted and smooth.
04 - Add softened butter, ground nutmeg, ground cinnamon, vanilla extract, and salt to the mixture. Stir until completely incorporated.
05 - Cover the bowl and refrigerate for at least 2 hours, until the mixture is firm enough to scoop.
06 - With a teaspoon or melon baller, scoop the chilled ganache and roll into 1-inch (2.5 cm) balls. Place on a parchment-lined baking tray. Refrigerate for 30 minutes to set.
07 - Melt the chopped dark chocolate in a heatproof bowl over barely simmering water (double boiler) or in short microwave bursts, stirring until glossy and smooth.
08 - Using a fork, dip each ganache ball into the melted dark chocolate, allowing excess coating to drip off. Return to the parchment-lined tray.
09 - Optionally, dust the tops of truffles with a pinch of nutmeg or cinnamon before the chocolate sets. Allow to set at room temperature or chill until firm.