It's called "xi bing." I had it for the first time recently when a family friend, whose daughter is getting married, gifted us with the taiwanese pastry. It's actually a Taiwanese tradition for the groom-to-be's parents to send xi bing to their close relatives/friends prior to the big day.
I'm told it's usually filled with five items: red bean, mochi, egg, dried pork ("ro song"), and oh darn, can someone tell me the other ingredient? It's surrounded by a thick pastry and topped with tons of sesame.
I've seen them come in small, single packages at bakeries like JJ Bakery, though the wedding version is usually one huge 'cake'. The trick to get the cake -- let the bakery know you want one, who will then make an 'extra' one when a wedding party orders a lot of them.