Heat the milk in a medium saucepan. Make sure it will hold at least a quart, as the mixture may foam up dramatically. Once the milk is hot, whisk in the chopped chocolate, stirring until it is completely melted. Add a pinch of salt.
For the best hot chocolate, bring to a boil, then reduce heat slightly and cook, whisking constantly, 2-3 minutes. Taste and add vanilla sugar (or regular sugar, if you prefer) to taste. Whisk furiously to get a bit of froth on the top.
Serve warm in small cups. Fluffy, melting marshmallows are always an option.
Notes
Can be made ahead. Let cool, store in a glass jar in the refrigerator. Rewarm before serving.