I came up with this recipe by accident when some mint icing I made tasted too "toothpastey". Add some unsweetened chocolate, and hey presto, you've got a delicious after dinner mint taste that goes well with any chocolate based cake! Mmmm...
Melt chocolate in microwave at a medium heat, stirring every 30 seconds until almost melted. It will continue to melt after being removed from the microwave, so don't overcook it. Leave that to cool for 5 minutes.
Beat butter/margarine with, 2 cups of confectioners sugar until fluffy. Add salt, mint extract, milk/cream, melted chocolate, and the remaining 2 cups of sugar; and beat on high speed until well blended.
This should give you a perfect piping consistency. Add more or less milk/cream to meet your particular cake decorating needs.