In a saucepan, whisk together cocoa powder, sugar, and cornstarch until well combined.
Pour in a small amount of milk and stir until a smooth, clump-free paste forms.
Gradually add the rest of the milk while stirring continuously to blend everything smoothly.
Place the saucepan over medium heat and bring the mixture to a gentle simmer, stirring constantly.
Continue to cook until the mixture thickens to a creamy, smooth consistency. The longer you simmer, the denser it becomes. For a classic creamy version, remove from heat once simmering begins. Also consider that as it cools down it will became denser.
Pour into mugs, add whipped cream if you like, and enjoy warm!
