That is one giant wooden block your sushi is sitting on. Is it special Japanese cherry wood? What's that? It's a cake? Oh very well then, you may proceed with your revelry as planned.
At first I thought this looked like the world's fanciest doorstop or something but then I started drooling and realized that my brain knew it was awesome food so why couldn't the rest of me figure it out? The only way to eat such a fancy dessert is in the tiniest nibbles possible. You have to be careful not to compromise the structural integrity of the stack.