01 - Place the finely chopped white chocolate in a heatproof bowl.
02 - Gently warm eggnog in a small saucepan over medium-low heat until steaming, without allowing it to boil.
03 - Pour the heated eggnog over the white chocolate and let stand for 2 minutes. Gently whisk until the mixture is completely smooth and fully melted.
04 - Stir in the softened butter, ground nutmeg, ground cinnamon, vanilla extract, and a pinch of salt until homogenous.
05 - Cover the bowl and refrigerate for at least 2 hours, or until the ganache is firm enough to scoop.
06 - Scoop the chilled ganache using a teaspoon or melon baller, and roll each portion into 1-inch balls. Arrange on a parchment-lined baking tray. Refrigerate for 30 minutes to set.
07 - Melt the dark chocolate in a heatproof bowl set over a pan of barely simmering water (double boiler) or in short intervals in the microwave, stirring until smooth.
08 - Using a fork, dip each ganache ball into the melted dark chocolate, allowing any excess to drip off. Place coated truffles on the parchment-lined tray.
09 - If desired, dust the tops with a pinch of ground nutmeg or cinnamon while the chocolate is still wet.
10 - Allow truffles to set at room temperature or briefly chill to firm the chocolate shell before serving or packaging.