Allow butter to soften.
Add sugar and cocoa.
Mix.
Add a little hot water or hot milk (rice, soy, almond, etc..,any kind will do)
Beat until smooth.
Ice and refrigerate to harden.
You can adjust the amounts of both sugar and cocoa to affect how thick and how chocolate-y you want the frosting to be.
Source of recipe: This is a spin I did off of my mom's traditional chocolate butter-cream she uses on German's chocolate cake. It's super easy and delicious.