In a pot, mix together the cocoa powder and sugar. (If you're using cinnamon, add it now.) Add 1/2 cup of water and whisk until the dry ingredients are completely dissolved into a syrup.
Heat over low heat, continuing to whisk, until the syrup is warm. (If you're using vanilla, add it now.)
Add the milk and turn up the heat to medium, and heat until it's hot. Don't turn it up too high, or the milk will suddenly rise up and boil over and it will make a horrible mess!