Step 1

Preheat oven to 350°. Lightly coat a 9x5" loaf pan with nonstick spray. Line pan with parchment paper, leaving a 2" overhang on long sides of pan; spray parchment.

Step 2

Spread hazelnuts on a small, rimmed baking sheet and toast, tossing occasionally, until fragrant and light brown, 7–10 minutes. Let cool 10 minutes, chop, and set aside.

Step 3

Bring cinnamon, cider, and sugar to a boil in a medium saucepan fitted with candy thermometer over high heat, swirling pan to dissolve sugar. Cook, swirling pan occasionally, until thermometer registers 280° and caramel is dark amber, 15–20 minutes.

Step 4

Remove pan from heat and add cream, butter, and 1 tsp. salt (mixture will bubble vigorously). Return to medium-high heat and cook, swirling pan occasionally, until thermometer registers 250°, 5–10 minutes. Remove from heat and swirl in Calvados, vanilla, and reserved hazelnuts.

Step 5

Pour into prepared pan and chill until firm but not hard, about 1 hour. Let sit at room temperature 5 minutes to loosen caramel, then lift from pan with parchment overhang. Peel parchment from caramel, trim edges, and cut into ¾" pieces. Wrap individually in parchment paper and chill, or serve on a platter. Sprinkle with salt just before serving.

Step 6

Do Ahead: Caramel can be made 2 days ahead. Tightly wrap and chill.