Combine cornflour with light soy sauce in a small bowl, stirring until fully dissolved, then incorporate the remaining sauce ingredients except for the water.
Pour 1½ tablespoons of this mixture over the prepared chicken, ensuring an even coating, and let it marinate for 10 to 20 minutes. Mix the reserved water into the remaining sauce.
Heat the oil in a wok over high heat. Sauté the garlic, ginger, and dried chillies until aromatic, about 30 seconds.
Add the marinated chicken and stir-fry until it begins to whiten. Incorporate the white sections of the green onions and continue cooking until the chicken is cooked through, approximately 2 more minutes.
Stir in the remaining sauce along with the Sichuan peppercorns. Simmer the mixture, stirring continuously, until the sauce thickens and reduces to a glossy consistency.
Fold in the peanuts and the green parts of the onions just before finishing. Taste and adjust the heat level with additional Sichuan pepper if desired.
Serve immediately with steamed rice, or opt for cauliflower rice for a lighter alternative.