In the bowl of a stand mixer. Cream together the softened butter and vegetable shortening. Beat for 3-4 minutes until fluffy and pale yellow in color.
If you'd prefer, you can use the exact same technique using all butter. In that case, it would be 4 sticks total. [The small amount of vegetable shortening stabilizes the frosting.]
Add the sifted powdered sugar alternately with the heavy cream.
If you'd like the icing to be easily spreadable for frosting cakes, add more cream one tablespoon at a time. If you'd like a more stiff icing to maintain it's shape for piping onto cupcakes, add slightly less.
After all of the powdered sugar has been added whip in the vanilla and salt. Adjust the mixer speed to medium-high and whip for 5-7 minutes until light and fluffy.
If you'd like to add color to this buttercream, I recommend using color gel additives so as not to change the consistency of the icing.
Makes enough tol frost  8/9" rounds for a layer cake or 30 cupcakes.
Buttercream icing prepared in this way will form a "crust" rather quickly. So, keep a damp cloth over the bowl of icing while you frost your cake or cupcakes to slow this process.