Put the milk, the cocoa powder, the sugar, the salt and optionally the cinnamon powder in a saucepan and bring to the boil, stirring from time to time with the whisk. The ingredients blend well when the mixture is hot enough.
Mix the cornstarch with a little milk, add to the saucepan and stir. After the mixture has boiled again, take it off the stove and serve in a cup.