The better the quality of chocolate used in these caramels, the more delicious the result. Caramels should be individually wrapped in cellophane or waxed paper the moment they are cut or they will absorb moisture from the air and begin to ooze.
Spray an 11 3/4-by-16 1/4-inch baking pan with vegetable-oil spray. Set aside in a spot where it will not be moved. In a heavy 4-quart saucepan, combine 2 cups cream, corn syrup, sugar, and salt. Clip on candy thermometer. Bring to a boil over medium heat, stirring occasionally with a wooden spoon, 15 to 20 minutes. Wash down sides of pan with a pastry brush dipped in water to remove any sugar crystals.
Cook, stirring constantly, until temperature reaches 220 degrees, 6 to 8 minutes; watch so mixture doesn't boil over. Continue stirring, and add chocolate and butter; keep mixture boiling, and slowly add remaining 2 cups cream. Cook, still stirring, until temperature reaches 240 degrees (soft-ball stage), about 60 minutes, keeping mixture at a low boil.
Without scraping pot, pour mixture into prepared pan. Let stand uncovered at room temperature for 24 hours without moving.
To cut, spray a cutting board with vegetable-oil spray. Unmold caramel onto sprayed surface. Using a large knife, cut into 1-by-1 1/4-inch pieces or other shapes. Wrap each piece in cellophane or waxed paper.