Line a medium sized rimmed cookie sheet with parchment paper, making sure to have paper go up and over sides. Butter it with 1 tablespoon butter.

Set pan aside in a spot where it will not be moved. In a heavy 4-quart saucepan, combine 1 cup 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 1 cup 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. Scrape pan sides and set aside to cool for an almost instant treat. 
    
Using a large knife, cut into 1-by-1 1/4-inch pieces or other shapes. Wrap each piece in cellophane or waxed paper.