Jamestown1630 posted a recipe or two, I did some searching of my own, and came up with this one I decided was the best (hand mixed no a mixer was used, interesting commentary from the poster and other commenters). I loved the rich dark color and combo of dutch process cocoa and cinnamon.
http://ths.gardenweb.com/discussions/2347412/spanish-bar-cake
The above is the link I found (I think I found it). The original link JT1630 posted was at taste of southern (clickable link on her post #11 I coded and referred to somewhere in this post).
To be authentic, it has to be cut lengthwise in a 9X13 pan or longer and stacked like a round layer cake and has to have the frosting just like as shown. And I would try to get the cake pretty flat. One way is to wrap wet paper towels with foil around the pan which is too much work and you can buy something on amazon which is too much money. I will try by first reducting the oven temperature just a little and turning the cake 180 every 10 minutes. Shrug. Just how I do it.
Finally, I had a terrible time finding when it first came up on FR and it was JT1630.
http://www.freerepublic.com/focus/chat/3356517/posts?page=11#11
IMO, there is a photo of one with the box on Justapinch showing the presumably authentic cake, and it is lighter than the one I posted; ingredients I didn't compare exactly.
But back then they didn't use cream cheese frosting I don't think but people are free to do as they like. And if I put raisins in it, I'm going to chop them up finer and maybe soak them, maybe not.
The poster of the recipe I chose just says vaguely to use a plain white icing or sprinkle with powdered sugar. So I guess I would recommend this one for frosting which I have used and don't remember how white it was:
Kittencal's Bakery Buttercream Frosting at food.com
I guess Kittencal's frosting is white enough, and it has the right fluffiness and texture as near as I can tell.
When I made it, I did use the half real butter for flavor and probably whipping cream but milk would do. I started with a scant 4 cups of SIFTED powdered sugar. Can't remember what I put it on or I'd post a photo. The cupcake was posted by somebody else at food.com
The subject has also been taken up at pinterest (just google pinterest spanish bar); I'm weary of coding links.