Combine masa harina and 1/2 cup cold water in a small bowl to form a paste.

In a medium saucepan, add the masa harina paste and the remaining 3 cups of water.

Bring to a boil, stirring constantly to prevent lumps.

Reduce heat and cook slowly for 20 minutes, stirring frequently until the mixture thickens.

In a separate saucepan, heat the milk until it comes to a boil.

Add the Mexican chocolate tablet to the milk and cook for about 10 minutes, stirring until the chocolate is completely dissolved.

Pour the chocolate mixture into the masa harina mixture, stirring until well incorporated.

Use a milinillo (or an eggbeater) to whisk the mixture vigorously until a foam forms on top, about 3 minutes.

Serve immediately in a large bowl, spooning some of the foam on top of each serving.