Line a 9x5x3-inch loaf pan with foil, extending the foil over the edges of the pan. Butter the foil; set pan aside.
Butter the sides of a 2-quart heavy saucepan. In saucepan combine sugar, half-and-half, chocolate, corn syrup, and salt. Cook and stir over medium heat until mixture boils. Clip a candy thermometer to side of pan. Reduce heat to medium-low; continue boiling at a moderate, steady rate , stirring occasionally, until thermometer registers 236 degree F, soft-ball stage (20 to 25 minutes.) (Adjust heat as necessary to maintain a steady boil).
Remove saucepan from heat. Add butter and vanilla, but do not stir. Cool, without stirring, to 110 degree F (50 to 60 minutes).
Remove thermometer from saucepan. Beat mixture vigorously with a clean wooden spoon until fudge just begins to thicken. If desired, add nuts. Continue beating until the fudge just starts to lose its gloss (6 to 8 minutes total).
Immediately spread fudge evenly in the prepared pan. Score into squares (or other shapes) while warm. When fudge is firm, use foil to lift it out of pan. Cut fudge. Store tightly covered up to 1 week. Makes about 1-1/4 pounds (32 pieces).