Add the cider to a 2 quart saucepan and bring to a boil over medium-high heat. Lower heat slightly but allow the cider to remain at a boil. Continue to cook, stirring occasionally, until it has reduced to a thick syrup, between 1/3 and ½ cup. Meanwhile, line an 8×8-inch baking pan with parchment paper.
When the cider is reduced to the correct volume, remove the pan from the heat and add in the butter, sugars and cream. Return the pan to the heat and let cook, stirring occasionally, until the mixture reaches 252˚ F. (Be sure to watch the temperature very closely. Don’t let Facebook or your smartphone ruin a perfectly good batch of caramels! Speaking from experience here.) As soon as the mixture reaches the correct temperature, remove from the heat, stir in the cinnamon and salt, and pour the mixture into the prepared pan.
Let stand until completely set, at least a few hours or overnight. Sprinkle with additional flaky sea salt if desired. Remove the slab of caramel from the pan, slice with a lightly greased sharp knife or pizza cutter into 1-inch squares. Wrap in small squares of parchment paper and twist the ends to seal shut. Store in an airtight container.