In a medium saucepan, heat the milk over medium heat until it starts to simmer. Add the cinnamon stick if using.
Meanwhile, mix the cornstarch with 3 tablespoons of cold milk to create a slurry.
Add the chopped chocolate to the warm milk and whisk until completely melted.
Pour in the cornstarch mixture while continuously whisking to prevent lumps.
Cook for 5-7 minutes, stirring constantly, until the mixture thickens to your desired consistency.
Remove the cinnamon stick and serve hot.
