This is a gua bao 刮包, or "Chinese hamburger." Stewed pork meat is put inside a steamed bun, then covered with some sour pickle and then, unfortunately, sugar powder. There are two ways to order this to make it great. First, you can choose between fatty and lean meat. Unless you enjoy chewin' on some yummy tender fat, order the lean meat--"shou rou." Also, ask for no sugar powder (tang fen 糖粉). This 刮包 is from a famous Taipei eatery, Lan Family Gua Bao: Lan Jia Gua Bao 藍家刮包. The phone number's in the photo. Hey, who said I never done nothin' for the peeples?
You can find them all over the place, though. Even sometimes in night markets here in Kaohsiung, you will find a guy with a cart pulling hot steamed buns from under a white cloth. As soon as people realize what they are selling, a line forms quickly.