Butter an 8-inch square baking pan; line with parchment paper, allowing a 1-inch overhang. Butter parchment paper, and set aside. Check the calibration of the candy thermometer. Put butter into a large bowl, and set over a wire rack; set aside. Put cream, sugar, chocolate, corn syrup, and salt into a medium heavy saucepan.

Cook over medium-low heat, stirring constantly, until sugar has dissolved and chocolate hasmelted, about 10 minutes. Raise heat to medium. Bring mixture to a boil, washing down sides of pan with a wet pastry brush to prevent sugar crystals from forming. Attach warm thermometer to pan; continue to cook, without stirring, until mixture registers 238 degrees (soft-ball stage), 10 to 15 minutes.

Immediately pour mixture into bowl with butter (do not scrape out the bottom of the pan). Attach thermometer to bowl. Let mixture cool, undisturbed, until it registers 110 degrees, about 1 1/2 hours.

Remove thermometer. Transfer bowl to a work surface. Using a wooden spoon, begin to gently stir mixture.

Continue to stir, gradually increasing speed until mixture thickens, lightens in color, and loses its sheen, 4 to 5 minutes.

Pour into prepared pan. Using a small offset spatula, quickly spread fudge to sides of pan, and smooth top. Let fudge cool in pan on a wire rack, about 1 hour. Cover with plastic, and refrigerate until completely set, about 8 hours. Run a sharp knife around nonparchment sides to loosen; lift fudge out. Cut into 64 pieces, each 1 inch square. Fudge can be refrigerated between layers of parchment in an airtight container up to 1 week; bring to room temperature before serving.