Remove butter from the refrigerator 30 minutes before starting. (You want it to be room temperature but not too soft.)
Using a stand mixer fitted with the wire whip, add butter to bowl and beat until smooth.
Add powdered sugar, cocoa powder, vanilla, salt, and milk. Beat until fluffy (about 2 minutes).
After cake is cooled, frost between layers and on top. Refrigerate cake immediately after spreading on frosting.