Beat the butter in the bowl of a mixer until fluffy. Add the powdered sugar and beat slowly until incorporated, increasing the speed to beat it more fully.
Scrape down the sides and the bottom of the bowl and add the cocoa powder, milk, vanilla, and salt. Mix on medium-high until completely smooth and combined. If the frosting is too stiff, add more milk one tablespoon at a time until it is a pipeable consistency.
Use this frosting to frost cupcakes, cakes, or whatever dessert your heart desires.