Passage: Many Hong Kong restaurants, such as cha chaan tengs and dai pai dongs, offer an item called a buttered pineapple bun, which is a pineapple bun with a piece of butter stuffed inside. They are known in Cantonese as boh loh yau (菠蘿油), in which boh loh means "pineapple", and yau (oil) refers to butter. Variants of this include using custard in place of butter.

Question: What is stuffed inside the pineapple bun?
butter