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Put sugar, corn syrup, water and salt into saucepan.
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Stir over low heat until sugar is dissolved.
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Increase heat and cook until thermometer registers 250 degrees F, firm ball stage.
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Add butter; cook with occasional stirring until thermometer registers 300 degrees F (brittle stage).
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Remove from stove.
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Add vanilla extract.
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Pour into buttered shallow pan, making a layer 1/4 inch deep.
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While warm, crease into squares with a blunt knife.
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When cold, break into pieces.