Rocket News 24 calls this marvelous cake a Baumkuchen. It’s popular in Japan, but originally from Germany. From the inside and outside, it’s a remarkable facsimile of a real watermelon, complete with seeds in the pulp.
It’s made on a spit. The baker adds layers of batter onto a spinning rod until it becomes a thick mound of cake. It has rings like a tree, which is why the German name for it means “tree cake.”