Combine water and cinnamon in a medium saucepan over medium-high heat. Bring to a boil and let it boil for 7 minutes.

Add the chocolate tablet, stirring constantly until fully dissolved, about 5–6 minutes. Reduce heat to medium and cook for 2 more minutes.

In a small container with a lid, combine cornstarch and cold water. Shake or stir until smooth to create a slurry.

Stir evaporated milk and sugar into the chocolate mixture. Add the cornstarch slurry and mix well.

Increase heat to a simmer and cook until the mixture thickens slightly, about 3 minutes, stirring frequently. Remove from heat and serve warm.