For the chile oil: Heat the oil, Sichuan peppercorns, cinnamon stick and star anise in a small pot over medium-low heat until it reaches 325 degrees F, then remove from the heat and let sit for 5 minutes. Remove the peppercorns, cinnamon stick, and star anise with a slotted spoon and discard, then stir in the crushed red pepper flakes. Set the oil aside to cool.
For the sauce: Whisk together the soy sauce, sesame paste, sugar, five-spice powder, Sichuan peppercorns, garlic and 1/4 cup of the chile oil in a medium bowl. Whisk in 1/3 cup hot water and set aside.
For the noodles: Bring a large pot of water to a boil. Cook the noodles according to the package directions, adding the bok choy, if using, for the last minute of cooking time. Drain in a colander and rinse well under cold running water to stop the cooking process. Toss the noodles with 1 teaspoon of the reserved chile oil and set aside.
Heat 3 tablespoons vegetable or peanut oil in a seasoned wok over high heat until shimmering. Add the pork and cook, stirring and breaking it up with a spoon, until it is browned, about 5 minutes. Add the Shaoxing wine, hoisin sauce and dark soy sauce and cook until all the liquid is evaporated and the pork is crispy, about 3 minutes. Add the sui mi ya cai, if using, and cook until it starts to sizzle, about 2 minutes. (Otherwise, remove from the heat after the pork is crispy.) Remove from the heat and add the reserved noodles and sauce to the wok, tossing well to coat. Place in a serving bowl, top with scallions and serve immediately.
Sui mi ya cai are chopped, fermented mustard greens. Find them online or at most Asian markets. They also go by the name ya cai. If none are available you can make the dish without them.
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