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, about 35 to 40 minutes.
Stir the cinnamon and flaky salt together in a small dish.
Line the bottom and sides of an 8-inch square metal baking pan with crisscrossed parchment.
Once the apple cider is reduced, remove it from heat and stir in the butter, sugars, and heavy cream.
Return the pot to medium-high heat with a candy thermometer attached and boil until the thermometer reads 252 degrees, about 5 minutes.
Remove caramel from heat, add the cinnamon-salt mixture, and stir to distribute evenly.
Pour caramel into the prepared pan and let it sit until cool and firm, about 2 hours.
Once firm, transfer the block to a cutting board and cut into 1-by-1-inch squares using a well-oiled knife.
Wrap each square in a 4-inch square of waxed paper, twisting the sides to close.
