In a large bowl, beat the softened butter with an electric mixer until creamy.
Add the cocoa powder and beat until combined.
Gradually add the sifted powdered sugar, alternating with the milk, beating on low speed until combined, then increase speed to medium-high and beat until light and fluffy.
Stir in the vanilla extract and salt.
If the frosting is too thick, add a little more milk; if too thin, add more powdered sugar.