In a large bowl using an electric mixer, cream butter and shortening until creamy and smooth. Add cocoa powder, vanilla extract, and salt; mix until combined.
Slowly add powdered sugar 1 cup at a time; beating well after each addition. Scrape down bowl as necessary.
Once all powdered sugar has been mixed in, add 1 tablespoon of milk at a time and mix. Continue to add and mix until desired consistency is achieved. Eat & Enjoy!
Notes
Store Chocolate American Buttercream in airtight container in refrigerator for 1-2 weeks. When you are ready to use, bring to room temperature and re-whip.