1 Tablespoon butter, softened (plus more for lightly buttering the pan)
coarse sea salt for topping
Line an 8×8″ pan with aluminum foil that overextends the pan on all sides (to lift out for easy removal). Lightly grease foil with butter.
Combine cream, sugars, and light corn syrup in a heavy-bottomed saucepan. Heat over medium heat, stirring constantly with wooden spoon until sugars are melted and mixture starts to boil.
Stop stirring. Attach a candy thermometer to rim of pan making sure the bottom of the thermometer doesn’t touch the bottom of pan. Let mixture boil until temperature reaches 245 degrees F (it’ll take time so be patient!) As soon as it reaches 245 degrees, remove from heat and add in salt and butter. Stir until butter melts.
Pour hot mixture into prepared pan and sprinkle coarse salt over top. Let set uncovered on counter for 4 hours or overnight.
Once set, remove caramel from pan by lifting the foil. Remove foil and with a sharp knife cut into rectangles or squares. Wrap individually in wax paper wrappers. Store at room temperature.