Line an 8 by 8 pan with foil, extending foil over edge of pan. Butter foil and set aside.
Butter the sides of a heavy 2 qt. saucepan. In saucepan combine sugar, milk, chocolate and corn syrup. Cook and stir over medium high heat until mixture boils. Attach candy thermometer to side of pan. Reduce heat to medium-low continue boiling, stirring frequently, until thermometer reaches 234 degrees F, soft ball stage 20-25 minutes.
Remove saucepan from heat. Leave thermometer in pan and add butter and vanilla, but Do Not Stir. Cool without stirring to 110 degrees F, about 1 hour.
Remove thermometer from saucepan. Beat mixture vigorously with a wooden spoon until fudge just begins to thicken. If desired, add nuts. Continue beating until the fudge becomes very thick and just starts to lose it's gloss, about 10 minutes total.
Immediately spread fudge in desired pan. Score into squares while warm. When fudge is firm use foil to lift out of pan. Cut fudge into squares. Store tightly covered. Makes a pound of fudge.
To make peanut butter fudge substitute 1/2 to 3/4 C creamy peanut butter for the cocoa powder and either use less butter or none at all.