Pour cream into a heavy saucepan, and swirl up all sides. Add sugar, butter, cream of tartar, and salt, and stirring constantly, bring to a boil over medium-high heat. Stir in corn syrup. Then wash down sides of pan and the spoon.

Cook, stirring occasionally until it reaches 240 °F., and stir in vanilla.

Remove from heat, and pour directly onto marble slab (do NOT scrape the sides of the pan). Allow to cool enough to touch and handle, and add vanilla. Beat and knead with a bench knife, wooden spoon, or your hands until it loses its shine, and becomes the texture of smooth fudge. Form into desired shapes, and wrap tightly with plastic.