In a stand mixer fitted with a paddle attachment, cream the butter. Start off at a low speed and then gradually increase until the butter is light in color and smooth. Use a rubber spatula to scrape the bowl as needed.
Add the powdered sugar, one cup at a time, mixing at a low speed until fully incorporated before you add the next cup.
Add the heavy cream (start with 2 tbsp), vanilla and salt and beat at a low speed for about 20 seconds. If you feel your buttercream is too stiff you can add the third tablespoon of cream. Increase the speed to high and beat for about 3-5 minutes until the mixture is perfectly smooth and fluffy. Stop the mixer once to scrape the bowl.
Stir the buttercream by hand to remove any air bubbles.