Combine the chicken with the dark soy sauce, light soy sauce, Chinese rice wine, white pepper, and cornflour in a bowl, then cover and marinate for 30 minutes. The chicken should look evenly coated with a thin, tacky cornstarch layer before frying.
Pour the neutral oil into a large frying pan, reserving 1 tbsp, and heat it over medium-high heat. The oil should shimmer and give a steady sizzle when the first piece of chicken touches it.
Fry the chicken in batches for 4 to 5 minutes, turning the pieces a couple of times as they cook. Keep the oil around 325°F to 350°F and look for a golden perimeter with crisp-looking edges.
Transfer the fried chicken to a plate once each batch is golden. Leaving space between pieces in the pan prevents steam from softening the cornstarch coating.
Heat the reserved 1 tbsp of oil with the sesame oil in a wok over medium-high heat. The sesame oil should smell nutty and fragrant without smoking heavily.
Add the onion and dried red chillies and stir-fry for 2 to 3 minutes. The onion should begin to soften at the edges while the whole chilies darken slightly and smell smoky.
Stir in the ground Sichuan peppercorns and cook for 1 minute. A strong citrusy aroma should bloom quickly, so keep the peppercorns moving to prevent scorching.
Add the garlic, ginger, and spring onions and stir-fry for 30 seconds. The wok should smell sharp, warm, and fragrant before the garlic takes on any color.
Pour in the light soy sauce, Chinese rice wine, chilli bean paste, and sugar, then stir-fry for 1 minute. The mixture should bubble and become a savory, lightly glossy coating rather than a thin pool.
Return the chicken to the wok and toss for 1 to 2 minutes until heated through. Every piece should look lacquered with sauce while the crisp coating remains visible around the edges.
Serve the chicken with boiled rice. The rice catches the spicy oils and savory glaze without competing with the peppercorn flavor.