Boil the apple cider in a large pot over high heat until it reduces to a dark, thick syrup, between 1/3 and 1/2 cup in volume. It should take about 35 to 40 minutes, and by the end, bubbles will begin to turn shiny, viscous, and pop more slowly.
While the apple cider is reducing, line the bottom and sides of an 8- inch straight- sided square metal baking pan with 2 long sheets of crisscrossed parchment. Set aside. Stir the cinnamon and flaky salt together in a small dish, and have the butter, sugars, and creme fraiche measured out and ready to go.
When the apple cider has reduced, remove it from the heat and stir in the butter, sugars, and creme fraiche, just until dissolved. Return the pot to medium-high heat with a candy thermometer attached to the side, and let it boil until the thermometer reads 252 degrees, only about 5 minutes. Keep a close eye on it.
If you don’t have a thermometer, prepare a bowl of very cold water while the caramel is boiling. Cook the caramel until a spoonful dropped into the water becomes firm, chewy, and able to be plied into a ball. You may want to cook the caramel at a lower heat for a little longer, to give yourself time to test the caramel before taking it off the heat.
When the caramel is at the right temperature or consistency, immediately remove it from heat, add the cinnamon-salt mixture, and give the caramel several stirs to distribute it evenly. Pour the caramel into the prepared pan. Let it sit at room temperature or in the fridge until cool and firm. At room temperature, it will take about two hours; in the fridge, one should do it.
Once caramel is firm, use the parchment paper to transfer the block to a cutting board. Use a well-oiled knife or a pizza wheel, oiling it after each cut, to cut the caramel into 1-by-1-inch squares. It will slice better when cold. Optionally, sprinkle each square with a bit of flaky salt. Wrap each one in a roughly 3×4-inch piece of waxed or parchment paper, twisting the sides to close.
If you don’t have crème fraîche, just sub in equal parts heavy cream.