I happened to have a chance to try a "peanut butter cup stout" last night!!! Brewed by Odd Side Ales: http://www.oddsideales.com/main/
The stout was very stouty in most regards except the tail end of the flavor was a rush of peanuts. I'm not sure how similar it is to eating a peanut butter cup, but hey it was very interesting. I would suggest emailing the brewery and just ask them if they use any extracts or anything.
If it were me, I'd shoot for getting that nutty flavor from some adjunct/specialty grains. Particularly, Honey and Victory malts may be right up your alley. See the write-up here: http://www.brew-monkey.com/brewschool/maltguide.php
Keep us posted if you end up trying it!