Generously butter or grease a large baking sheet, and set aside.
In a saucepan over medium heat, combine the butter, sugar, salt and corn syrup. Cook, stirring until butter is melted. Stir in the cashews, and increase the heat to high.
Heat to 300 to 310 degrees F (149 to 154 degrees C), or until a small amount of syrup dropped into cold water forms hard, brittle threads. Stir occasionally with a long handled wooden spoon so the cashews do not burn.
Pour onto a greased baking sheet, and allow to cool for about 10 minutes. Break into pieces, and store in an airtight container lined with waxed paper.
