01 - In a large bowl, beat the softened butter with an electric mixer on medium speed until creamy and smooth, about 1 minute.
02 - Sift together the powdered sugar, cocoa powder, and salt.
03 - Gradually add the dry mixture to the butter, mixing on low speed until combined.
04 - Pour in the milk and vanilla extract; beat on medium speed until the frosting is light and fluffy, about 2 to 3 minutes.
05 - If the frosting is too thick, add more milk, one tablespoon at a time, until desired consistency is reached.
06 - Use immediately to frost cooled cupcakes, or store covered in the refrigerator for up to 3 days. Bring to room temperature and re-whip before using.