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