In a large bowl, combine cream cheese and butter. Using an electric mixer fitted with the whisk or paddle attachment, cream together until well combined and smooth (about 1 to 2 minutes on medium speed). You can do this by hand, but it's much MUCH easier with the mixer.
Add powdered sugar and mix together on low until fully dissolved. Add cocoa powder, salt, and vanilla, and mix again on low until combined.
Whip the frosting for 3 to 4 minutes on high speed, or until light and fluffy. If the frosting is too thick, add 1 to 2 tablespoons of milk, or until you reach your desired consistency. If the frosting is too soft, chill for 30 minutes or until firm enough to use.
