Combine 2 cups water and cinnamon in a saucepan over medium-high heat. Bring to a boil; let boil for 7 minutes. Drop in chocolate tablet and stir until dissolved, 5 to 6 minutes. Reduce heat to medium and cook for 2 minutes more.
Combine cornstarch and cold water in a small container with a lid; shake until mixed well.
Stir evaporated milk and sugar into the chocolate mixture in the saucepan. Add cornstarch mixture and stir well. Increase heat to a simmer and cook until slightly thickened, about 3 minutes.