Combine the cream, corn syrup, and salt in a medium saucepan and bring to a boil. Remove from the heat and add the chocolate, swirling the pan so that the cream completely covers the chocolate. Let sit undisturbed for about 5 minutes, then add the sour cream, butter, and vanilla and whisk until mixture is smooth and shiny and chocolate is completely melted. Set aside, stirring every so often to help the chocolate cool. Once the mixture thickens to spreading consistency, use it to frost cakes as desired.