1Combine the sugar, corn syrup, and 1/2 cup of cream in a large saucepan set over moderate heat.
2Stir the mixture until the sugar dissolves completely.
3Insert a candy thermometer, reduce the heat to low, and cook the syrup, stirring frequently, until the thermometer registers 238.
4Blend in another 1/2 cup of the cream, which will cause the temperature to drop, and continue to cook and stir until the thermometer reaches 236 or until a bit of the hot toffee dropped into a little cold water forms a soft, pliable ball.
5Mix in the remaining 1/2 cup cream and the melted chocolate.
6Cook the toffee, stirring constantly, lest it scorch.
7Cook toffee until the mixture becomes quite thick: A drop of it should firm up quickly in cold water.
8NOTE: Even though the temperature of the toffee may not exceed 230, if it firms in cold water, remove the pan from the heat at once.
9Quickly mix in the vanilla and salt, then pour the toffee into a well-buttered 8x8x2 inch pan.
10Cool the toffee completely, then cut it into 1-inch squares.