Beat the softened butter in a large bowl until smooth and creamy, about 2-3 minutes.
Sift in the cocoa powder to prevent lumps, then gradually add the powdered sugar, one cup at a time, mixing well after each addition.
Add the vanilla extract and salt, and mix to combine.
Slowly add the milk (or heavy cream), one tablespoon at a time, until the frosting reaches your desired consistency.
Beat the frosting on high for 2-3 minutes until it’s light, fluffy, and smooth.