In a bowl, combine the chicken pieces with 2 tablespoons soy sauce, rice vinegar, and 1 teaspoon cornstarch. Mix to coat and let marinate for 10 to 15 minutes while you prep the rest.
Whisk together the hoisin sauce, 1 tablespoon soy sauce, water or stock, and sugar in a small bowl to create the glossy kung pao sauce. If you like more heat, add chili flakes to this mixture.
Heat a large skillet or wok over medium-high heat and add 1 tablespoon oil. When shimmering, add the marinated chicken in a single layer. Let it sear without touching for 2 minutes, then stir-fry until light golden and cooked through. Remove to a plate.
Add the remaining 1 tablespoon oil to the pan. Toss in the minced garlic and grated ginger, cooking until fragrant (about 30 seconds). Add the bell peppers and stir-fry for 2–3 minutes until vibrant and slightly tender.
Return the chicken to the pan. Pour in the prepared sauce and toss to coat everything evenly. Stir in the roasted peanuts and let the sauce simmer and thicken for 1–2 minutes.
If the sauce is too thick, add a splash more stock or water to reach your desired consistency. Finish with chopped green onions and, if using, lightly crushed Szechuan peppercorns for a citrusy, numbing note.
Taste and adjust: a bit more soy for saltiness, a touch more sugar if you like a sweeter balance, or extra chili for more heat. Serve hot, ideally with steamed rice to soak up any extra sauce.