Line an 8x4x2-inch loaf pan with foil, extending foil over edges of pan. Butter foil; set aside.

Butter the sides of a heavy 2-quart saucepan. In saucepan combine sugar, evaporated milk, and corn syrup. Cook and stir over medium-high heat to boiling. Carefully clip a candy thermometer to side of pan. Cook over medium-low heat to 238 degrees F., soft-ball stage, stirring frequently. This should take 20 to 25 minutes.

Remove saucepan from heat. Add butter and vanilla, but do not stir. Cool without stirring to 110 degrees F (about 45 minutes). Remove candy thermometer from saucepan. Carefully stir butter into cooked mixture to incorporate. Beat mixture vigorously with a wooden spoon until fudge becomes very thick (about 8 minutes total). Immediately spread fudge into prepared pan. Score into squares while warm. If desired, top each square with a nut piece. When candy is firm, use the foil to lift it out of pan. Cut into squares. Makes about 1 pound (32 servings).

Prepare and cut fudge as above. Place in an airtight container and store in the refrigerator for up to 3 days.