Dry and cut the beans: Trim the stem ends and cut the beans into 2- to 3-inch pieces. Spread them on a clean kitchen towel, pat them dry, and leave them for about 10 minutes while you prep everything else. Any water left on the beans will fight the blistering in the next step.
Blister the beans: Heat a wok or large cast-iron skillet over high heat until it just begins to smoke. Add 2 tablespoons of the oil and swirl to coat. Add half the beans in a single layer and cook, undisturbed, for 1 minute, then toss and spread them out again. Keep cooking, tossing every 30 to 60 seconds, for 5 to 7 minutes total, until the skins are wrinkled and blistered with browned spots and the beans are crisp-tender. Move them to a plate. Repeat with the second half, adding a drizzle more oil if the pan looks dry.
Crisp the pork: Pour off all but about 1 tablespoon of oil from the pan and return it to medium-high heat. Add the ground pork, press it into a thin layer, and let it brown undisturbed for 2 minutes before breaking it into small bits. Cook 2 to 3 more minutes, until the pork is deeply browned at the edges and cooked through. If you are using mushrooms instead, cook them the same way until their liquid cooks off and the edges brown.
Bloom the aromatics: Push the pork to one side. Add the garlic, ginger, Sichuan peppercorns, dried chilies, preserved vegetable, and doubanjiang (if using) to the cleared space. Stir for 30 to 45 seconds, until the oil turns red and fragrant, then toss everything together. Pour the Shaoxing wine around the edge of the pan and let it sizzle off for a few seconds.
Bring the beans back: Return the blistered beans and any juices on the plate to the pan. Add the soy sauce, sugar, and salt. Toss over high heat for 1 to 2 minutes, until every bean is glossy and coated with the aromatic bits. Taste a bean: it should be savory first, then warm, with a light tingle building at the end. Add a pinch more salt or soy sauce if it tastes flat.
Finish and serve: Take the pan off the heat. Stir in the toasted sesame oil if you are using it, and shower the scallions over the top. Serve right away, while the beans are still wrinkled and the pork bits are crisp.