01 - Place the finely chopped white chocolate in a heatproof bowl.
02 - Warm eggnog in a small saucepan over medium-low heat until steaming but not boiling.
03 - Pour the hot eggnog over the white chocolate and let stand for 2 minutes.
04 - Gently whisk until the chocolate has melted and the mixture is smooth.
05 - Mix in the softened butter, nutmeg, cinnamon, vanilla extract, and a pinch of salt until well blended.
06 - Cover the bowl and refrigerate for at least 2 hours or until the ganache is firm enough to scoop.
07 - Using a teaspoon or melon baller, portion and roll the chilled ganache into 1-inch balls and place on a parchment-lined baking tray.
08 - Refrigerate the rolled balls for 30 minutes to help them set.
09 - Gently melt the chopped dark chocolate in a heatproof bowl over a pan of barely simmering water or in short microwave bursts, stirring until smooth.
10 - Using a fork, dip each ganache ball into the melted dark chocolate, letting excess drip off. Transfer to the parchment-lined tray.
11 - If desired, dust the coated truffles with extra nutmeg or cinnamon before the chocolate hardens.
12 - Allow truffles to set completely at room temperature or briefly chill to firm the chocolate shell.