Boil the apple cider in a 3- to- 4- quart saucepan over high heat until it is reduced to a dark, thick syrup, between ⅓ and ½ cup in volume. This takes about 35 to 40 minutes on my stove. Stir occasionally.
Meanwhile, line the bottom and sides of an 8-inch straight-sided square metal baking pan with 2 long sheets of crisscrossed parchment. Set it aside. Stir the cinnamon and flaky salt together in a small dish.
Once you are finished reducing the apple cider, remove it from the heat and stir in the butter, sugars, and heavy cream.
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.
Immediately remove caramel from heat, add the cinnamon-salt mixture, and give the caramel several stirs to distribute it evenly.
Pour caramel into the prepared pan. Let it sit until cool and firm—about 2 hours, though it goes faster in the fridge.
Once caramel is firm, use your parchment paper sling to transfer the block to a cutting board. Use a well-oiled knife to cut the caramel into 1-by-1-inch squares.
Wrap each one in a 4-inch square of waxed paper, twisting the sides to close.
