A while back a young Vietnamese-American woman emailed about how her boyfriend was in love with a particular kind of steamed bao filled with a buttery yellow filling. Did I know how to make them? She would be so thrilled to be able to gift him one of his favorite sweets. I felt like the bao competed with her for his affection. I was compelled to help her but did not have time to until now.
I first thought that the bao was filled with a sweetened mung bean filling, which is buttery and somewhat marzipan like, and directed her to a recipe in Asian Dumplings. No, she replied, the filling that she was looking for tasted eggy.
Then I realized that she was looking for egg custard buns, called lai wong bao (or nai wong bao) in Chinese, which translates into English as "milk custard bun". That awkward name sounds more like a problem spot that needs lots of exercising than a steamed leavened bun filled a rich egg-yolk and sugar filling.
I recently sampled lai wong bao from a San Jose Vietnamese-owned bakery that tasted of egg yolk and a touch of coconut milk. It wasn’t cloying and had a nice richness and touch of tropical Asia. That rendition of egg custard bao got me researching. Today, I made a batch.
Custard Powder and Other Key Ingredients
One of the key ingredients to the filling is custard powder, a popular British baking ingredient that is used in a number of Hong Kong-style sweets. (Hong Kong was a British colony for many years, which explains why the custard powder is popular.) Custard powder is mostly made of cornstarch with some salt, vanilla, and a touch of ground annatto for color. The go-to, popular brand is Bird’s Custard Powder, which you can find specialty foods stores and some Chinese markets.
Vanilla pudding mix is a worthy substitute. I suppose you could also sub plain cornstarch and add ground annatto, but formulating a knock-off version of custard powder was not part of my Sunday morning plans!
Despite the cornstarch in the custard powder, there's lots of additional corntarch in the filling to make it thicken up and cohere. Old fashioned egg custard buns include salted duck egg yolks to lend a savory hit to the sweet filling. I opted to add salt but had a good amount of rich coconut milk. Some cooks use evaporated milk or dry milk powder but I find that sweetened condensed milk lends better depth and flavor. The photo above captures the ingredients that I pulled out for the filling. The rest is egg yolk and butter.
Texture and Tweaking Tips
Some renditions of egg custard buns are somewhat runny when you break open the bun whereas others are more solid but very soft. I went for the soft-firm side of things with the filling below. It is easier to shape into a ball and manage as you are encasing it in the dough. That required lots of cornstarch and the filling had a slight starchiness that I didn’t care for. My husband didn’t mind it at all and said, “What needs to be changed? It tastes great!”
Nevertheless, if you want to tinker with the filling, reduce the amount of cornstarch to 2 1/4 ounces, maybe even 2 ounces. Don't worry as the filling will be plenty thick; see the Note on how to handle it if its too soft to shape. After pulling the thickened mixture off the heat, let it cool a tad, taste it, then adjust it. If you want the filling on the runny side, then stir in coconut milk by the tablespoon. If you add too much liquid, recook the mixture to thicken it up. The filling seems to be very elastic, and you can bend it to your will.
So, to the young woman who got me going on making egg custard buns: I hope your relationship is going well and will get even better with homemade lai wong bao!
Egg Custard Bun Filling
Lai Wong Bao
As with all bao fillings that I make, this one was used along with 1 1/4 pounds of the basic yeast dough on page 92 of the Asian Dumplings cookbook. Follow the instructions for the steamed filled buns recipe (page 95) to shape and cook the buns. For the ones pictured at theh top of this post, I used bleached all-purpose flour for a lighter colored bao dough. The dough and yellow filling of the egg custard bun make it look like a boiled egg!
The amount of filling made here is a little bit less than the usual 1 1/3 cups so you will have extra dough. If you like, twist the extra dough off after you’ve closed the bun up. I made 16 medium buns as they are of moderate size, neither dainty nor gargantuan.
4/22/11 Update: In addition to my comments above, several people have added terrific comments below on adjusting the amount of starch to avoid a gritty finish. Please take a read to help you make an educated guess on how much to use. This filling is relatively flexible so you can play with it.
Makes a scant 1 1/4 cups
1/4 teaspoon salt
2 tablespoons custard powder
1/2 cup sugar
About 2 1/2 ounces (1/2 cup) cornstarch
1/2 cup coconut milk
2 tablespoons sweetened condensed milk
2 tablespoons unsalted butter, cut into 3 or 4 pieces
2 large egg yolks
1/2 to 1 teaspoon vanilla extract
1. In a saucepan, whisk together the salt, custard powder, sugar, and cornstarch. Whisk in the coconut milk and condensed milk. Heat over medium heat, stirring constantly, until smooth. Add the butter and continue stirring until the butter has melted.
Keep stirring for about 6 minutes, until the mixture has thickened substantially. If it seems hard to stir with the whisk, switch to a spatula. As the mixture stiffens it may look like there are lumps. If you see that, stir more vigorously to break the lumps up and evenly cook. Aim for a smooth texture.
2. When the mixture is thick enough to stick to the whisk and form a soft peak, move the pan off the heat and whisk in the egg yolks. This will thin the mixture out slightly for the moment. Return the pan to the heat and keep whisking to ensure that the yolks are well incorporated.
Continue vigorously stirring for 1 to 2 minutes more, until the mixture thickens to the point where it cleans the sides of the pan and easily comes together as one smooth mass. If you have ever made dough for cream puffs, this is very similar. Stir in the vanilla, then take the pan off the heat. Stir it occasionally for a few minutes to quickly cool it.
At the end, stir the mixture off the heat for a minute and taste it. Make any changes you like at this point, as I suggest above. When you're satisfied, transfer the filling to a bowl, spreading it out.
3. After the filling has cooled, form it into balls.For my 16, each one was about 1 1/4 inches wide, like an extra- large or jumbo egg yolk! Loosely cover and set aside at room temperature, or refrigerate to firm up, until your dough is ready. Then use the balls of filling when you shape the buns. Do the second rise and steam as directed in the steamed filled bun recipe in Asian Dumplings.
Should the filling be too soft after it has cooled, portion it into generous tablespoons, dropping it onto parchment. Loosely cover with plastic wrap and refrigerate to firm up. I found that you can leave it in the fridge until you’re ready to shape the bao. However, a cold filling slows down the second rising so be patient.
Related Posts: Chicken and Shiitake Mushroom Bao filling (on Asian Dumpling Tips)