In a small bowl, dissolve the cornstarch and the soda in two cups of cow's milk.
Heat the remaining 10 cups of milk in a very large heavy bottomed saucepan and bring to a boil. Add the cornstarch and soda mixture to the boiling milk. Add the sugar and cinnamon stick and continue cooking, stirring with wooden spoon, until the mixture thickens and you can see the bottom of the pan clearly. This process will take about one hour, during which time the mixture will cook down.
Cool and store in covered jars.
Cajeta will keep indefinitely if refrigerated.
You may use all cow's milk if goat milk is not available.