First, in a large pot, bring the apple cider to a boil over medium heat. Reduce to a simmer and let it cook down until it’s reduced to about 1 cup. The "right consistency" will be like syrup. This should take an hour and will concentrate the apple flavor. Stir occasionally to prevent burning.
Second, take the reduced cider off the heat, add both sugars and butter, and stir until melted and combined.
Third, slowly pour the heavy cream into the sugar mixture while stirring.
Then, return to the heat and bring the mixture to a boil, and cook for 2 minutes or until it reaches 225 degrees using a candy thermometer.
Next, remove the mixture from the heat and add cinnamon and salt.
Place parchment paper in a large sheet tray and pour the hot caramel onto it evenly to cool, and sprinkle with flaky sea salt.
Place the caramel in the fridge for two hours or leave it at room temperature for 4 hours.
Finally, spoon the caramel into a glass jar or container to store. Be sure to warm before serving.