First: make sure your butter and cream cheese are softened at room temperature.
In a stand mixer, cream together the butter and cream cheese until light yellow and fluffy. Scrape down the sides as needed in between mixing.
Add the vanilla and salt; mix.
Then gradually add the powdered sugar until the frosting is light and creamy.
Video
Notes
Makes about 2 cups - this is enough for a two-layer cake that feeds 8 to 10 people. If your frosting is too thin: add more powdered sugar, one tablespoon at a time, until the frosting is to your preferred consistency.If your frosting is too thick: add more milk, one teaspoon at a time, until the frosting reaches your preferred consistency. Storage: store frosting in the refrigerator in an airtight container for up to one week.