Melt the chocolate in a double boiler over low heat. In a medium saucepan, bring the cream and milk to a bare simmer. In a large bowl, whisk together the sugar, vanilla, salt and egg yolks.
Slowly pour the milk mixture into the egg mixture to temper it, whisking all the while. Return the mixture to the saucepan and cook over low heat until the mixture coats the back of a spoon, about 4 minutes. Pour in the melted chocolate and whisk until smooth.
Pour into glasses or custard cups. Refrigerate until chilled and set, about 3 hours. Serve with whipped sour cream, if desired.