Place all ingredients in a saucepan over medium heat. Let the mixture come to a boil, watching closely. When it starts to rise and you see a chocolate foam forming, turn off the heat. Immediately pour the hot chocolate into cups, filling each cup halfway first to distribute the foam evenly, then fill the cups the rest of the way. Serve hot.

Tip: For best results, add the sugar to the mixture while heating. This helps keep the foam on top of the hot chocolate and prevents it from disappearing when you stir in sugar later.