These tasty little guys must surely have come to Indonesia by way of China. There, they're generally stuffed with sweet red bean paste or barbecued pork. Here the classic versions contain mung bean paste (called "green bean" or "kacang hijau"), or the inevitable bits of chicken. Some places sell a chocolate version but I haven't gotten my hands on one yet.
Good steamed buns have a sticky, aggressively bland dough. There's a kind of spongey mushiness to them that I love. They are sold everywhere in Jakarta from little carts like these.