Heat vegetable or peanut oil in a large skillet or wok over medium-high heat.

Cook pork belly until lightly browned on the bottom, about 3 minutes.

Flip pork belly and cook until lightly browned, about 2 minutes longer.

Transfer pork belly to a plate and set aside.

In the same skillet, add garlic and ginger and cook over medium-high heat, stirring, until fragrant.

Add star anise and chili (if using), and sugar and cook, stirring, until the sugar is melted and bubbling, about 2 minutes.

Add rice wine and bring to a boil, stirring until sugar dissolves, about 2 minutes.

Add five spice powder, dark and light soy sauces, and water and bring to a boil.

Return pork belly to the skillet and reduce heat to low.

Cover skillet and cook until pork belly is very tender, at least 1 hour or preferably 2 hours.

Meanwhile, combine peanuts and sugar in a blender or food processor and pulse until the mixture resembles a coarse powder, about 1 minute.

Set a steamer over a pot of boiling water.

Add buns, cover steamer and cook until buns are heated through and fluffy, about 3 minutes.

Spread pickled mustard greens inside each steamed bun.

Set a piece of pork belly on top of mustard greens.

Top pork belly with a pinch of the chopped cilantro and a sprinkle of the peanut powder.

Serve immediately.