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1
In a heavy 2-quart saucepan combine the brown sugar, the corn syrup, the butter, the vinegar, and the water, bring the mixture to a boil over moderate heat, stirring and washing down any sugar crystals clinging to the side with a brush dipped in cold water, until the sugar is dissolved, and boil the syrup, undisturbed, until it is deep golden and a candy thermometer 290F.
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2
Stir in the vanilla, pour the mixture into a buttered 13- by 9-inch metal baking pan, tilting the pan and spreading mixture with a wooden spoon, and let the toffee cool completely.
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3
In the top of a double boiler set over barely simmering water melt the chocolate chips, stirring until the chocolate is smooth, and spread the chocolate evenly over the toffee.
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4
Sprinkle the cashews evenly over the chocolate, pressing them gently into it, and chill the confection for 30 minutes, or until the chocolate is firm.
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5
With a thin knife loosen the confection from the pan and, using the knife as a lever, remove it in one piece from the pan.
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6
Cut or break the toffee into serving pieces.
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7
The toffee keeps, separated by sheets of wax paper in an airtight container lined with wax paper, in a cool dry place for 2 weeks.
