Melt chocolate in a double boiler. If you do not have a double boiler use a bowl set over a saucepan of simmering water.
One the chocolate is melted, remove from heat and let it cool slightly.
Combine sugar and salt in a medium sized bowl.
Beat butter pieces into the sugar salt combination until thoroughly mixed. This may take a few minutes.
Then slowly add the chocolate and sour cream, alternating, until it is completely mixed.
At this point the frosting is ready to use.
However, if you feel it is too soft you can chill it in the refrigerator until it is thickened. Be sure to stir it from time to time.
Frosting should be used right away or you can put it into an airtight container and keep it in the fridge for about 4 days.
If you do make it ahead of time you will want to let it come to room temperature before using and you may even find that you need to mix it with the mixer if it is lumpy.