4cups heavy cream, divided
2 1/2cups light corn syrup
4 1/2cups granulated sugar
1teaspoon salt, plus more for sprinkling as desired
10ounces high quality bittersweet chocolate (30 to 40%)
8ounces high quality very dark chocolate (70 to 80%)
8ounces (2 sticks) unsalted butter, cut into cubes
2teaspoons vanilla extract
Line a 12 x 16-inch baking pan with a Silpat or parchment paper and spray generously with nonstick spray. Place the pan in a spot that is easily accessible, but where it will not be moved (the caramel will wrinkle if you move it before it has cooled completely).
In a large pot, combine 2 cups of the cream with the corn syrup, sugar, and salt (be sure to use a large pot — cream has a tendency to boil over!). Clip a candy thermometer to the side of the pot (an infrared laser thermometer is pretty great, too), and bring it to a boil — stirring occasionally — over medium heat. Wash down the sides of the pot as needed with a pastry brush dipped into tepid water.
Cook, stirring constantly, until the temperature reads 220° F on the thermometer. Continue stirring, and add the chocolate and butter (you may need to adjust the temperature to keep it at a rolling boil). Once the mixture has returned to a boil, gradually add the remaining 2 cups cream.
Continue to boil, stirring constantly, until the temperature reads 240 to 245° F on the thermometer. Stir in the vanilla.
Pour the caramel into the prepared pan (garnish with a sprinkling of additional salt, if using) and let sit — undisturbed and uncovered — for at least 12 hours and up to 24 hours.
When the caramel has cooled completely, spray a cutting board and knife with nonstick spray. Unmold the caramels onto the cutting board, and peel away the Silpat or parchment paper. Cut into rectangles or squares. Wrap in wax paper or parchment and store in a cool, dry place.