Add apple cider to a sauce pan and bring to a boil. Simmer until apple cider has reduced by half. This could take 10-20 minutes depending on the size of your sauce pan.
Add in brown sugar and butter mix until butter has melted. Bring to a simmer and add in heavy cream and cinnamon. Bring to simmer for 5 more minutes stirring frequently.
Add in vanilla extract and flakey salt. The sauce will thicken more as it cools.