01 - Place the 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 the hot eggnog over the white chocolate and let stand for 2 minutes, then whisk gently until smooth and fully melted.
04 - Stir in unsalted butter, ground nutmeg, ground cinnamon, vanilla extract, and salt until incorporated.
05 - Cover the bowl and refrigerate for at least 2 hours, or until the ganache is firm enough to scoop.
06 - Scoop out the chilled ganache using a teaspoon or melon baller and roll into 1-inch balls. Arrange on a parchment-lined tray.
07 - Refrigerate the rolled ganache balls for 30 minutes to allow them to firm before coating.
08 - In a heatproof bowl, melt the chopped dark chocolate over a double boiler or with short microwave bursts, stirring until completely smooth.
09 - Dip each ganache ball into the melted dark chocolate using a fork, letting any excess chocolate drip off, and return to the parchment-lined tray.
10 - Lightly dust truffles with extra ground nutmeg or cinnamon before the chocolate coating sets, if desired.
11 - Let truffles set at room temperature or place briefly in the refrigerator until the outer shell is firm.