Place a large pot over medium heat. Pour in the water, add in the cinnamon stick. Cut open the vanilla bean, scrape off the seeds. Add the beans and pod into the pot. Also, add in the chocolate and let it melt. Keep an eye on it, we don't want it to burn. Then, mix to dissolve in the water. 

When the mix reaches a low simmer, add in the 4 cups of milk and let it reach a simmer once again. 

Meanwhile, in a bowl whisk the masa harina with the 2 cups of milk. Its not going to dissolve and thats ok, but be sure to break up any lumps.  Set aside until needed. 

The chocolate mix should be simmering by now. Gradually and slowly pour in the masa/milk mixture, while whisking to incorporate. Whisking will help break down the chocolate even more. Add sugar to taste and stir to dissolve. Let it reach a gentle simmer once again and cook it for 10-15 minutes or until it slightly thickens. Whisk/stir as needed or continuously. 

When ready, turn off the heat. Serve in a a mug and ENJOY! Be careful when serving the champurrado, as its consistency will preserve it really hot.