I found this over on instructables.
More examples here - http://www.flickr.com/photos/27064046@N00/sets/72157603857787202/ and http://www.dhreck.com/2008/12/10/reddit-alien-pixel-cookie/
Too cool! Have you tried them yet?
When I was a kid we made checkerboard cookies that were cut out by hand and stacked, but I haven't tried this extrusion method yet.