Cigar City makes an Oatmeal Raisin Cookie brown ale. It's their Maduro brown ale with vanilla, raisin puree, and cinnamon added to secondary fermentation.
It's only on draft, but I was able to try it and it tasted just like an oatmeal cookie. I think the raisins gave it the cookie taste more so than the vanilla and cinnamon.
Maybe adding a small amount of raisins (whatever way you decide) would work. Don't do so much that you get a strong flavor from it, but a subtle one that adds to the cookie taste. Maybe combine that with vanilla bean and cocoa power or something along those lines.