01 - Place the chopped white chocolate into a heatproof bowl.
02 - Warm the eggnog in a small saucepan over medium-low heat until steaming, taking care not to let it boil.
03 - Pour the heated eggnog over the white chocolate. Allow to sit for 2 minutes, then whisk gently until completely smooth and melted.
04 - Mix in the softened butter, ground nutmeg, cinnamon, vanilla extract, and a pinch of salt until fully combined.
05 - Cover the bowl and refrigerate the ganache for at least 2 hours, or until it is firm enough to scoop.
06 - Use a teaspoon or melon baller to scoop portions of the chilled ganache, rolling them into 1-inch balls. Arrange on a parchment-lined baking tray and refrigerate for 30 minutes to set.
07 - Gently melt the dark chocolate in a heatproof bowl set over a pan of barely simmering water, or microwave in short bursts, stirring until smooth.
08 - Using a fork, dip each ganache ball into the melted dark chocolate, allowing the excess to drip off before placing back on the lined tray.
09 - Optionally, sprinkle the tops of the coated truffles with a pinch of ground nutmeg or cinnamon before the chocolate sets.
10 - Leave the truffles to set at room temperature or briefly chill to firm up the coating.