I've been asked to do this for a friend. I can pretty much figure it all out aside from the outside edge; the fluted/rippled part. It looks like it was put on at the end, rather than poured at the same time as the top. But what on Earth?? I'm no expert by any means so perhaps it's something easy.... but you all are so great at these things I wanted to check with you first... before I tell her I can do it. 
Here is the link to the one she wants:
http://www.williams-sonoma.com/products/perfect-endings-peanut-butter-cup-cake/
Here is the link to the one she wants:
http://www.williams-sonoma.com/products/perfect-endings-peanut-butter-cup-cake/








