Using a stand mixer or handheld mixer, mix together the butter, cocoa and salt until smooth. Then add in the powdered sugar and beat on low. Add in the vanilla and 1 tablespoon of milk. Add more milk ½ tablespoon at a time if needed for desired consistency. Mix for 1-2 minutes longer, until smooth and fluffy.
Spread evenly on brownies. (This frosting is enough to frost an 8x8 pan of brownies.)