Using an electric mixer, cream together butter and cream cheese until fluffy.
Add in salt and vanilla extract and beat for additional minute.
Stir together the powdered sugar and cocoa powder and add to mixer in 1 cup increments until fully incorporated.
Notes
For looser frosting, add milk or heavy cream in 1 tablespoon increments. For thicker frosting, add powdered sugar in 1/4 to 1/2 cup increments until you've reached desired consistency.