In a large bowl, mix powdered sugar and cocoa powder together; set aside.
In the bowl of your mixer, beat butter, vanilla, and cream on medium low speed until smooth and combined.
Decrease mixer speed to low and slowly add powdered sugar mixture, beating on low until barely mixed. Increase speed to high and beat for about a minute and a half, until smooth, light and fluffy. A butter knife should easily move through frosting -- if too thick, add cream, 2 tablespoons at a time until desired consistency is reached.