So... which end do you start from?
Here in Hong Kong, a bun like that can have almost anything in it: red bean paste, plain custard, desiccated coconut mixed with custard, curry minced beef, satay minced beef, roast pork in sweet BBQ sauce (char siu), chicken and mushroom with cream sauce, sausage, corned beef, spam and egg, ham and corn salad, tuna…