In a medium saucepan, mix all ingredients and melt over medium heat until sugar is completely dissolved.
Without further stirring, boil until mixture reaches 290°F on a candy thermometer.
Pour mixture into a greased 9" x 12" pan and let cool until almost firm to the touch.
Score surface into 2" squares with a sharp knife, but do not cut through completely. Allow to cool completely.
When cool, break into pieces (this should be easier if toffee was properly scored).