Pour the water (or water and milk) into a medium saucepan. Add the sugar and canela stick., Set the saucepan over high heat and bring the liquid to a boil, stirring to dissolve the sugar., Reduce the heat to medium and add the chocolate pieces, stirring to incorporate the chocolate as it melts., Weigh your masa harina; or measure it by gently spooning it into a tablespoon, then sweeping off any excess., Reduce the heat to low and whisk in the masa harina a little at a time, stirring constantly until the mixture is smooth., Continue to heat the champurrado over low heat, whisking regularly, until it’s silky and thickened, 12 to 15 minutes., Remove the champurrado from the heat, discard the canela stick, and serve as is or with a dollop of whipped cream., Refrigerate any leftover champurrado in an airtight container for up to three days. Reheat over low heat in a saucepan or in the microwave, adding water or milk as needed for your desired consistency.