Using a stand mixer or hand mixer, beat butter, cocoa powder, vanilla and salt together until smooth in a large bowl. Add powdered sugar in two additions, beating until smooth each time, scraping the sides of the bowl as needed.
Slowly beat in milk, a tablespoon or two at at a time, until frosting reaches desired consistency. Frosting should be light and fluffy and easily spreadable.
Use frosting as desired!
Video
Notes
If you use evpaorated milk, you can store this buttercream at room tempearture for 3 days. If you use whole milk, refridgerate any leftover buttercream.