This hot chocolate is one of those fond memories from childhood. In Spain, this drink is often sold at breakfast time with churros. Home versions are eaten with toasty bread.
The consistency is thicker than other hot chocolate drinks. It should stick to the spoon, but not be as heavy as a pudding.
The chocolate needs to be broken down. It can be shaved by hand or a quick couple of pulses in the food processor or blender. Pieces should not be much bigger than a grain of rice. This is so melting will be smooth and not scorched nor seized up.
Pour milk, sugar, and add chocolate to a heavy pan. Heat over medium stirring constantly to keep from burning. Once chocolate starts to bubble, turn down heat and let simmer. Cover 15 minutes.
Check consistency. Chocolate should stick to the spoon but still pour. If too thick, add more milk and heat to a boil while stirring. If too thin, turn up heat and stir until desired thickness is achieved. Some people will also add cornstarch at this phase if too thin.