Place the chicken in a bowl. Add all marinade ingredients, mix well, and set aside.
Prepare the vegetables: slice onion into chunks, garlic and ginger thinly, and cut dried chillies into 1 cm pieces.
Mix the kung pao sauce ingredients: combine the potato starch slurry with soy, black vinegar, and sugar. Set aside.
Heat 4 tbsp oil in a wok over high heat. Toast the cashew nuts for 2 minutes until lightly golden. Remove and set aside.
In the same wok with remaining oil, add dried chillies, Szechuan peppercorns, onion, and ginger. Stir-fry for 30 seconds.
Add the marinated chicken and stir-fry for 3–4 minutes until nearly cooked.
Pour in the prepared kung pao sauce, then add toasted cashews, spring onions, and garlic. Stir-fry for 2 minutes until the sauce thickens and coats the chicken evenly.
Serve immediately with steamed jasmine rice.