I used this frosting this weekend inside of a chocolate cake for my 14 year old's birthday party (chocolate peanut pave). It was a hit. I did use crunchy instead of creamy because I wanted some texture and it was delicious.
Place the butter and peanut butter into a medium bowl, and beat with an electric mixer. Gradually mix in the sugar, and when it starts to get thick, incorporate milk one tablespoon at a time until all of the sugar is mixed in and the frosting is thick and spreadable. Beat for at least 3 minutes for it to get good and fluffy.