We're told Kue Lumpur, or mud cakes, were a specialty of the Sidoarjo area even before the mud volcano. They're basically very tasty, eggy coconut custards. I'm not sure how they got their name.
We bought them at a Giant Hypermart in Surabaya, next to some Large Fried Things.
People at work laughed when I passed around "mud cakes from Sidoarjo." Some actually recoiled. This perhaps validates the fear of one distressed local, who said, "Sidoarjo used to be famous for mud cakes; now it's just famous for mud."