I used chunky peanut butter because that's what we had in the house. I was nervous about it, but in the end I wouldn't have had it any other way. If I were you whipping up this cake, I'd use chunky. It adds a whole new element to the cake.
Since this was a birthday cake, I decided to go with some festive sprinkles for decoration.
FYI this cake was eaten in less than 24 hours. It's that good. Seriously. And who doesn't love recipes that start with cake mix?