In a large bowl, beat the softened butter with an electric mixer until light and fluffy, about 2-3 minutes.
Gradually add the powdered sugar 1 cup at a time, beating well after each addition.
Add the heavy cream or milk, vanilla extract, and salt. Beat on high speed for 3-5 minutes until the buttercream is light, fluffy and smooth.
Use the buttercream immediately or store in an airtight container in the refrigerator for up to 1 week. Bring to room temperature before using.
