1. Add cocoa to a large bowl or bowl of stand mixer. Whisk to remove any lumps. Cream together butter and cocoa powder until well combined.
2. Add sugar and milk to cocoa mixture by adding one cup of sugar followed by about a tablespoon of milk. After each addition has been combined, turn mixer to a high speed for about a minute. Repeat until all sugar and milk have been added. Add vanilla extract and combine.
3. If frosting appears too dry, add more milk (a tablespoon at a time until it reaches the right consistency). If it appears too wet and does not hold its form, add more confectioners' sugar (a tablespoon at a time until it reaches the right consistency).