1. 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.

2. 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.

3. Without scraping pot, pour mixture into prepared pan. Let stand uncovered at room temperature for 24 hours without moving.

4. 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.