Butter should be softened at room temperature first; butter should not be too soft or runny. If that occurs, place into the refrigerator for 5 to 10 minutes. Place butter into a mixing bowl and whisk for about 5 minutes on high speed, until butter is light, fluffy and pale white in color.
Add the remaining ingredients: vanilla, salt and heavy cream. Gradually add in the confectioner's sugar, mixing on low speed first, then on high speed. Add more or less sugar, depending on desired consistency. Once all sugar has been added, whisk buttercream on high speed for a few more minutes, until fluffy.
If desired, additional flavors may be added at the very end. Some recommend flavors: 1/4 cup strawberry or raspberry puree; 1 tablespoon lemon zest; 1 tablespoon instant coffee + 2 tablespoons water.