To make the marinade, combine all of the ingredients together in a bowl. Add the chicken and mix, then cover and leave in the fridge for 1-2 hours to marinate.
Toast the black and Sichuan peppercorns in a wok or frying pan over low heat until fragrant. Allow to cool, then crush with a mortar and pestle.
Combine the fish sauce, dark soy sauce, oyster sauce, Shaoxing wine and 2 tablespoons water in a bowl, and set aside.
Heat enough oil for deep-frying in a wok or heavy-based pan. Add the basil leaves and fry them briefly, just until they stop sizzling. Remove and set aside on paper towel to drain.
Stir the chicken through the marinade once more, then carefully deep-fry in batches for around 30 seconds. Remove and set aside.
Discard all but 2 tablespoons of oil from the wok and heat over high heat, until smoking. Add the crushed peppercorns and stir-fry briefly, then return the chicken to the wok with the fish sauce mixture and toss to coat in the sauce.
Stir-fry over high heat until the sauce thickens and the chicken is cooked, 2-3 minutes. Add the sliced red chilli and stir through.
Transfer the chicken to a plate and sprinkle with the fried basil leaves. Serve with steamed rice.