2Combine sugar, butter, water, and salt in a heavy saucepan. Cook, stirring continuously until the mixture reaches light crack stage (310 degrees F, 160 degrees C) on a candy thermometer). Remove the pan from heat and stir in vanilla. Pour mixture into the prepared baking pan.
3Lay chocolate bars on top of the hot mixture and spread the melting chocolate over the top of the mixture. Chill well. Crack into pieces to serve.