The trick to making this somewhat heavy icing light is whipping in lots of air. Make sure you beat it at a high speed for 7 to 10 minutes to achieve a light consistency.
Cream butter and cream cheese in a stand mixer. Add salt and vanilla. Slowly start to add powdered sugar at low speed.
Once all the powdered sugar is incorporated, turn mixer to high and whip for at least 7 minutes or longer, scraping sides. If the icing seems too thick, mix in a tablespoon or 2 of the milk or cream.
Source: New York Times Cooking