Pineapple buns’ name sake figuratively describes the boxy design on the crusty top. They are usually plain with no filling. If they come with filling, it would be a sweet custard, a sweet red bean paste, or even a savory dried flakey pork filling. They look very similar to Mexican sweet rolls, except that it incorporates a Chinese food principle of yin and yang, a crusty top (yang) and soft inside (yin).