Lava Custard Bun (Liu Sha Bao): Salted Egg Yolk Custard

A lava custard bun, lau sa bau (流沙包) in Cantonese, is a small steamed bun with a molten centre of salted egg yolk custard. Break it open while it is hot and a golden, gently grainy filling flows out, sweet first and then quietly salty. It is one of the most photographed items on a modern dim sum table, and, unlike most of its neighbours, it is a recent invention rather than an old classic.

A newcomer to the dim sum table

Most familiar dim sum dishes have roots that reach back generations. The lava custard bun does not. It is generally described as a Hong Kong creation of the late twentieth century, built on the older custard bun, nai wong bau (奶黃包), whose firm, sweet filling of egg, milk and sugar has been a dim sum staple for much longer.

Precisely when and where the flowing version first appeared is not well documented, and claims from individual restaurants are hard to verify. What is clear is that by the early 2000s it had spread widely through Hong Kong and southern China, and from there to Cantonese restaurants abroad, including Vancouver's.

The name says a great deal. Lau sa means "flowing sand", a description of the filling's texture: the salted yolks give it a fine, sandy richness, and the butter makes it run when warm.

What makes the filling flow

The key ingredient is the salted duck egg yolk, cured until firm and deeply savoury. The usual method runs like this:

• The yolks are steamed or baked, then mashed or sieved into a fine paste.
• The custard is built with butter, sugar and milk or custard powder, sometimes with coconut milk.
• The filling is chilled until solid, so it can be portioned and wrapped.
• The bun is a soft, slightly sweet leavened dough, sealed tightly around the frozen or chilled filling.
• The steam melts the butter-rich centre while the dough rises around it.

Because the filling is solid when wrapped and liquid when served, the bun must be well sealed. A good one holds its shape on the plate and opens with a slow, even flow.

Mangosteen buns and playful shapes

Alongside the classic round bun, many kitchens now shape their custard buns to look like fruit, animals or other playful forms. Making dim sum that imitates something else is an old Cantonese craft in its own right, but these whimsical designs belong to the past decade or two, when social media rewarded anything that looked as good as it tasted.

A mangosteen-shaped bun is a good example: typically a round, deep purple bun crowned with a little green calyx, styled after the tropical fruit. It is the same idea, a soft bun around a warm custard centre, dressed for the occasion.

How to eat it

The filling is the hottest part of the bun, so a little patience pays.

• Wait a minute after the steamer basket arrives.
• Open it over the plate. Tear a small gap or take a small first bite to let steam escape.
• Share carefully with children, making sure the centre has cooled enough.
• Pair it with tea. The richness sits well with a pot of jasmine or pu'er.

Lava custard buns at Neptune Palace

We serve two versions, one at each location, both listed as dessert item 902. At our Burnaby restaurant at The Amazing Brentwood, the Salted Egg Yolk Custard Buns (黃金流沙包) are the classic golden kind. At our Vancouver restaurant at Marine Gateway, the Mangosteen-Shaped Custard Buns (山竹流沙包) take the fruit-shaped approach. Both are recent additions to our menus and made in-house.

They are a lovely way to end dim sum, or to win over a younger guest at the start. Dim sum is served every day until 3pm; for weekend tables, calling ahead is recommended.

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