Butter sides of heavy 2-quart saucepan. In it combine sugar, milk, chocolate, salt, and corn syrup. This is where I also add the Fluff. Heat and stir over medium heat till sugar dissolves and mixture comes to boiling. Then cook to soft-ball stage (234~), stirring only if necessary. Immediately remove from heat; add butter and cool to lukewarm (110~) without stirring. (I let set in sink of cold water.) Add vanilla. Beat vigorously until fudge becomes very thick and starts to lose its gloss. Quickly spread in buttered shallow pan. Score while warm, cut when firm.
