1. Place the cocoa, chips, and salt in a medium bowl.

2. Bring the heavy cream to a simmer, either in the microwave or in a small pan on the stove, and pour it over the chocolate.

3. Stir until the mixture is smooth, add the vanilla, and set aside to cool to room temperature.

4. In a large mixing bowl, cream together the butter, shortening, and half of the confectioners'sugar until fluffy. It will go through a crumbly stage; keep on beating and it will come together.

5. Add half of the chocolate mixture and beat until it's incorporated.

6. Scrape the bowl, then add the remaining chocolate and confectioners' sugar.

7. Beat again until the filling is a fluffy, spreadable texture; adjust the consistency with a little more cream, if necessary.