Cook sugar in a dry heavy saucepan over moderately low heat, stirring slowly with a fork, until melted and pale golden. Continue to cook until deep golden. Remove from heat and carefully add heavy cream (mixture will vigorously steam and lumps may form, this is okay). Return to moderately low heat, stirring, until caramel lumps are dissolved. This may take a few minutes, but it will happen. Remove from heat and whisk in chocolate until smooth. Stir in sour cream.