Line a 9x5x3-inch loaf pan with foil, extending foil over edges of pan. Butter the foil; set pan aside.

Butter the sides of a heavy 2-quart saucepan. In the saucepan, combine sugar, half-and-half or light cream, unsweetened 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 to boil at a moderate, steady rate, stirring occasionally, until thermometer registers 234 degrees F, soft-ball stage (20 to 25 minutes). Adjust heat as necessary to maintain a steady boil.

Remove pan from heat. Add butter and vanilla, but do not stir. Cool, without stirring, to 110 degrees F (50 to 60 minutes).

Remove thermometer from pan. Beat mixture vigorously with a clean wooden spoon until fudge just begins to thicken. If desired, add chopped nuts. Continue beating until fudge just starts to lose its gloss (6 to 8 minutes total).

Immediately spread fudge evenly in the prepared pan. While warm, score fudge, into 1- to 1-1/2-inch rounds or squares. For rounds, use round cutters.

When fudge is firm, use foil to lift it out of pan. Cut fudge into rounds or squares. If desired, drizzle pieces with melted white chocolate. Top with nut pieces dipped in additional melted white chocolate and cooled on waxed-paper lined baking sheet until set. Makes about 1-1/4 pounds (32 pieces).

Store in an airtight container at room temperature for up to 1 week.