Cinnamon syrup is a type of simple syrup that is infused with cinnamon sticks. Cinnamon syrup is one of the two ingredients in Don's Mix (along with grapefruit juice). Cinnamon syrup has a warm spicy note that can mimic oak-aged spirits.
To make cinnamon syrup, make a standard simple syrup, and add 3-5 cinnamon sticks immediately after the syrup is off the heat. Cover the saucepan and let steep for 30 minutes to an hour, then strain the syrup into a clean bottle or jar.