Combine the chicken, 1/4 cup soy sauce, 1 tablespoon rice vinegar, 1 tablespoon peanut oil, 2 smashed garlic cloves, and 2 smashed slices ginger in a large bowl; season lightly with salt and pepper, cover, and refrigerate to marinate 1–2 hours.
Heat vegetable or peanut oil in a wok or deep skillet to 375°F (use a thermometer). While oil heats, set up dredging: place buttermilk in one bowl and flour in another.
Dip marinated chicken pieces in buttermilk, then coat evenly with flour. Fry in batches until golden and crisp, about 4–5 minutes per batch. Drain on paper towels and keep warm.
In the wok over high heat, wipe out excess oil, add 2 tablespoons oil, then add the dried chilies, sliced scallions, 2 chopped garlic cloves, chopped ginger, and orange zest; stir-fry about 1 minute until fragrant.
Pour in 1/2 cup soy sauce, 1½ cups chicken stock, 2 tablespoons rice vinegar, and 1 tablespoon honey; bring to a boil.
Whisk 2 tablespoons cornstarch with 2 tablespoons cold water until smooth. Add the slurry to the boiling sauce in up to two additions, stirring, until the sauce thickens.
Add the fried chicken to the thickened sauce and toss to coat; cook until heated through, about 2 minutes. Garnish with sliced scallions and cilantro leaves.
Cook the dried Chinese egg noodles according to package instructions, drain, rinse under cold water, shake off excess water, and pat dry.
Heat 1–2 tablespoons canola oil in a large nonstick or seasoned skillet over high heat. Add the noodles, spreading to cover the pan, and fry until golden brown and crispy, about 8 minutes, turning once and breaking up near the end. Transfer to paper towels and season with salt.
Discard excess oil from the pan and wipe clean. Whisk together 1/2 cup vegetable broth, 3 tablespoons oyster sauce, 1 tablespoon soy sauce, 2 teaspoons cornstarch, and 1 teaspoon sugar; set aside.
Heat the skillet over high heat with a little oil if needed. Add 1 heaping tablespoon minced ginger, 3 minced garlic cloves, and the minced scallion; stir-fry 30 seconds until fragrant.
Add the sliced onion, celery, rehydrated sliced shiitake mushrooms, and mixed vegetables; stir-fry until crisp-tender, about 2 minutes. Add the water chestnuts and push ingredients to the sides to form a well in the center.
Pour the reserved broth mixture into the center, bring to a boil, and stir to coat the vegetables. Stir in the bean sprouts, season with freshly ground black pepper to taste, then remove from heat.
Transfer the chow mein vegetable mixture to a warm platter, top with the fried crispy noodles, and serve the Hunan chicken alongside with extra soy sauce if desired.
