01 - Place finely chopped white chocolate in a heatproof bowl.
02 - Warm the eggnog in a small saucepan over medium-low heat just until steaming; do not allow to boil.
03 - Pour the hot eggnog over the white chocolate and let stand for 2 minutes; whisk gently until fully melted and smooth.
04 - Stir in the softened butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until thoroughly incorporated.
05 - Cover the bowl with plastic wrap and refrigerate for a minimum of 2 hours, until the mixture is firm enough to scoop.
06 - Using a teaspoon or small melon baller, scoop the chilled ganache and form into 1-inch balls. Arrange on a parchment-lined tray and refrigerate for 30 minutes to set.
07 - Melt the chopped dark chocolate in a heatproof bowl set over a saucepan of barely simmering water or microwave in short intervals, stirring until smooth.
08 - Using a fork, dip each ganache ball into the melted dark chocolate; allow excess chocolate to drain off before placing back onto the parchment-lined tray.
09 - Lightly dust each truffle with additional nutmeg or cinnamon if desired before the chocolate sets.
10 - Allow truffles to set at room temperature or chill briefly to firm the chocolate shell before serving or storing.