In a large bowl, beat the softened butter until creamy and smooth. Sift the cocoa powder into the bowl, and continue to beat until fully combined. Gradually add powdered sugar, one cup at a time, beating well after each addition. Pour in the milk a little at a time until the frosting reaches your desired consistency. Add vanilla extract and a pinch of salt, and mix until smooth and fluffy. If the frosting is too thick, add a little more milk; if it’s too thin, add more powdered sugar.