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