Lau Sa Bao (Steamed Buns with a Molten Salted Egg Yolk Filling)

Variations

• Nai wong bao (奶黃包): the same bun with a real custard in it — egg, milk and starch cooked thick over heat. Lau sa bao sometimes borrows "nai wong" for its name, but its filling is not a custard. A custard is egg that heat sets, and it is cooked to hit the window eggs describes, with yolk starting to set around 68°C. This filling never uses that window.
• The yolk left whole: mooncake and lo mai gai press one salted yolk into the middle unbroken. As the lo mai gai page puts it, "It concentrates the salt and the fat into one spot". Lau sa bao takes the same yolk, sieves it and beats it into butter so it runs through the whole filling instead.
• Baked: the same filling wrapped in a bread dough and baked, or in a cookie dough and pressed in a mould. Baked versions flow less than steamed ones, so warm them briefly before eating.
• Piglets and goldfish: dim sum houses colour and shape the dough. The filling works the same way.

Tips

• The salt has already set the yolk. Century egg says of the salt-cured duck egg: "Instead of going dark and creamy the yolk turns orange and hard." A yolk set like that never goes creamy again. So this filling does not try to melt the yolk. It cooks it right through into a powder and adds something that melts instead: the butter.
• What a mooncake avoids, this does on purpose. Mooncake only pre-bakes its salted yolks lightly: "Cooked through now, plus the main bake, and they crumble." In a mooncake the crumbling is a fault. Lau sa bao wants exactly that crumbled yolk. The sieved grains stay suspended in the butter without dissolving, and that is the "sand" you feel in your mouth.
• Wrap it cold, eat it hot. Xiaolongbao seals in a set stock jelly that melts into hot broth as they steam. Lau sa bao runs the same sequence with butter in place of gelatine: from the freezer the filling is a solid you can wrap, and in the steamer it becomes a liquid. Try to wrap it half-set and it melts in your hand, butter gets on the seal, and the seal opens as it steams.
• The same dough, minus the baking powder. Char siu bao says the rest of the bun family comes from its dough with only the filling changed, and "The rule about setting the filling first holds for all of them." That rule holds here. But char siu bao is meant to burst open in three at the top, and "that split is not luck — it comes from using yeast and baking powder together and steaming over very hard heat." In this bun that split is the way the filling escapes. So this recipe leaves out the baking powder and steams at medium heat.
• Re-steam them cold. Once cool the butter sets and nothing flows. Five minutes in a steamer keeps the dough softer than a microwave does.
• If it tastes too salty, add milk powder, not sugar. More sugar makes it both sweeter and just as salty. Milk powder adds bulk, which dilutes the yolk.