The ingredient famously absent from a pineapple bun is pineapple.
The Hong Kong-style pineapple bun, or bo lo baau, is named for appearance rather than flavor. Its cracked, golden-brown sugar crust resembles the patterned skin of a pineapple, while the soft bread underneath is typically a mildly sweet milk-bread-style dough. Traditional versions contain no pineapple fruit or pineapple juice.
The bun became especially associated with Hong Kong bakeries and cha chaan tengs. Its exact origin is uncertain, but documented Hong Kong production dates back at least to the 1940s. A popular variation called bo lo yau places a thick slice of butter inside the hot bun; despite the name, “yau” refers to butter, not pineapple oil.
Some modern bakeries do add pineapple, custard, red bean paste, coconut, or other fillings. Those are variations, however, and should not be confused with the traditional bun. Japanese melonpan and Korean soboro bread have similarly crisp, patterned toppings but are distinct pastries.