Cream 6 tablespoons unsalted butter for a few minutes with half (1 ½ cups) of the powdered sugar. It should be very smooth at this point before adding the rest.
Add ¾ teaspoon vanilla, ¼ teaspoon salt and 5 ounces semisweet chocolate, melted and warm (not hot), and mix well.
Add the remaining 1 ½ cups powdered sugar, alternating with 2 tablespoons of cream, until very smooth and creamy and of spreading consistency. Add the remaining 1 tablespoon of cream if necessary, a teaspoon at a time, depending on how you’re going to use it.
Keep it refrigerated and covered or in an airtight container if not using right away.
Notes
Have the ingredients at the correct temperatures:this is very important! Make sure the butter is softened and the chocolate warm, not hot, so it all integrates better.Sift the powdered sugar, always! It can clump during storage and will be hard to integrate otherwise.