An hour before starting, set out 8 tablespoons (1 stick) of butter at room temperature.
Use an electric mixer to cream the room-temperature butter, cocoa, and salt, starting on low and then increasing to medium speed. Add the powdered sugar and mix on low. Add in the milk and vanilla extract.
Increase the speed to medium-high, and beat the frosting until smooth and fluffy. Spread it on cooled brownies.
If you're not using the frosting right away, refrigerate it in a sealed container for up to one week. You also can freeze it for up to three months. Thaw it overnight in the refrigerator, bring it to room temperature and beat it again until fluffy.
Notes
To frost a 2-layer cake, use 1.5 times the ingredients.For a 3-layer cake, double the ingredients.