Mix cocoa and sugar in a saucepan.
While stirring, add hot water. Place over medium heat and, continuing to stir, bring to a boil.
Pour in the heated milk and, reducing the heat a little, warm the mixture, but do not bring it to a boil. Remove from fire.
Using a whisk, whisk the drink for 20 seconds. before the formation of foam. Pour into warmed cups. When you whisk cocoa, this allows, firstly, to prevent the formation of a film on its surface, secondly, to mix all the components well, and thirdly, the flavor of the drink is enhanced.