Add the milk to a medium saucepan and warm over low heat until steaming but not simmering.
Add the chopped dark chocolate and whisk continuously until completely melted and smooth.
Whisk in the cocoa powder if using, keeping the heat low.
Slowly whisk in the eggnog, stirring constantly. Do not allow the mixture to boil, as high heat can cause curdling.
Remove the saucepan from heat. Stir in the vanilla extract, nutmeg or gingerbread spice, and salt. Taste and adjust seasoning if needed. Stir in espresso powder or alcohol if using.
Serve warm with whipped cream, shaved chocolate, or a sprinkle of nutmeg.