Pour cider into a large heavy-bottomed pot. Using a Popsicle stick or wooden chopstick, mark the stick at the level of the cider; remove the stick and mark again at 1/2, 1/4 and 1/8 of the amount between the bottom of the stick and your first mark. These marks will serve to help you gauge when your cider has reduced to between 1/7th and 1/8th of the original amount.

Bring cider to a boil over high heat and then reduce to medium-low and simmer, uncovered, until reduced, approximately 2-3 hours. Check the level against your stick measure at regular intervals; overcooking will result in a syrup that is too thick (if that happens it can be remedied with the addition of some water).

Cider is finished when it gets just above the 1/8th mark and should coat the back of a spoon. Remove from heat and strain if desired. I use butter muslin, a more tightly woven form of cheesecloth that works much better than the fabric you find at the grocery store but if that is what you have just layer several layers together to most effectively strain the liquid.

Pour into a container and refrigerate. Cider syrup can be canned but I never do; it has a long shelf life in the fridge and ours is long gone before it could ever go bad!

Combine bourbon and cider syrup over ice and stir gently. Twist lemon zest and drop into drink, stir again, and serve.