Combine the sugar, corn syrup, and 1/2 cup of cream in a large saucepan set over moderate heat.
Stir the mixture until the sugar dissolves completely.
Insert a candy thermometer, reduce the heat to low, and cook the syrup, stirring frequently, until the thermometer registers 238.
Blend 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.
Mix in the remaining 1/2 cup cream and the melted chocolate.
Cook the toffee, stirring constantly, lest it scorch.
Cook toffee until the mixture becomes quite thick: A drop of it should firm up quickly in cold water.
NOTE: 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.
Quickly mix in the vanilla and salt, then pour the toffee into a well-buttered 8x8x2 inch pan.
Cool the toffee completely, then cut it into 1-inch squares.