Add water and cinnamon stick in a medium-sized saucepan and bring to a boil. Lower heat and simmer for about 4 minutes. Add oats, stirring frequently. When oats have absorbed most of the water, add in the milk and stir to combine well. Simmer until atole reaches desired consistency. Atole is usually on the thinner side for sipping. Add more milk if needed. Add piloncillo to taste. Remove cinnamon stick and serve.