I'm not much help with the cake part, but I have an awesome peanut butter butter cream recipe. I made dark chocolate brownies and iced them with this recipe. It was a huge hit! I used chunky peanut butter because that's what I prefer and it turned out perfect.
So I know they aren't on the healthy side of things but a friends birthday is coming up and she asked me to make cupcakes.... she loves peanut butter and a few years ago I was at a party where they had Peanut Butter CupCakes with Peanutbutter Icing.... does any one have a recipe for this type of thing?