I'm not sure who the first person to stuff sweet and fatty tender braised chashu-style pork belly into a lily-white steamed Chinese bao bun was, but they've certainly taken off like gangbusters. Pretty much every ramen shop in the city features a variation on the theme, from the braised Niman Ranch pork buns with hoisin, scallion, and cucumber at Momofuku Noodle Bar to the charred pork neck buns with sweet Japanese mayo at Hide-Chan.
It's a natural fit, since all these ramen shops have braised pork on hand for their ramen anyhow; all it takes is a tender steamed leavened bun and a couple condiments, and bam, instant profit.
Lucky for us, now that we know how to make our own chashu pork at home and steamed folded bao are readily available from any Chinese market, making these at home is ridiculously easy.
After you've made the pork, the first step is reheating it. Braised pork belly is tender and gelatinous, making it nearly impossible to slice. Instead, I slice it after it's completely cool in order to get neater, cleaner slices. For pork buns, I like my slices thicker than for ramen, but not massively so. About 1/4-inch is enough.
Reheating them by simmering them in leftover broth helps to make sure that they stay moist, as well as adding extra flavor that rides up into the cracks and crevices of the meat. It'll also play a crucial role in helping crisp and caramelize the meat later on.
The best part of pork belly buns is that great contrast between soft bun and crisp pork. There are a couple of ways to achieve it. The most straight-forward is to use the broiler, making sure that it's blazing hot before adding the pork as close to the broiler element as possible.
It'll spit and sputter a bit as it cooks, but don't worry, it'll soon learn how to behave properly.
The alternative method is to pull a Marsellus Wallace and get medieval on its a%&. This method has the advantage of being way faster and easier than the broiler method. It has the disadvantage of requiring a blowtorch.
A blowtorch gets much hotter than a broiler element does, which not only allows you to heat your pork faster, but it'll also create a much greater flavor and textural contrast within your bun as the pork belly blisters, bubbles, and burns under the intense heat of the flame.
As for the buns, you can make your own, but I find the pre-made ones in the refrigerator case at the Chinese grocer to be more than adequate provided they're reheated properly. Steaming in a bamboo steamer set over a wok works, and is a good method if you plan on having a big party and want to keep a constant supply warm. For a smaller number of servings, however, the microwave is the tool of choice.
By placing the buns on a plate, covering them with a damp paper towel or damp clean kitchen towel, and microwaving for just a minute or so, they come out as hot, puffy, and steamy as the best fresh-baked bao.
There are so many ways to top pork belly buns that I don't even know where to begin, but I personally like to think of my toppings in three discrete elements: a fresh vegetable, a sweet-tart pickle, and a creamy sauce. In this case, I've gone with scallions, pickled bean sprouts, and a slathering of spicy mayo.