Cut boneless chicken into 2-cm (1-inch) pieces. In a mixing bowl, combine marinade with chicken pieces and let marinate for 30 minutes.
Cut dried chillies diagonally into big sections (using a pair of kitchen scissors), and remove the seeds. Dip into cold water and immediately drain. Set aside.
In a mixing bowl, combine sauce ingredients and stir until sugar dissolves.
Heat up 1/2 cup of oil in a wok over medium heat until moderately hot (about 160 - 170 deg C or 320 - 340 deg F). Fry chicken chunks in two batches, until chicken pieces are cooked and lightly browned. Scoop out with a perforated ladle and let drain on paper towels. Set aside. Drain the wok of oil, wash and dry the wok.
Heat up 4 tbsp of oil in the wok over low heat. Fry the Sichuan peppercorns until fragrant, scoop out and discard (or set aside half for later). Fry dried chillies in the same oil until chillies expand and turn reddish brown in colour. Scoop out and set aside.
In the same oil, fry garlic slices, minced ginger and chopped spring onion briefly until fragrant. Add sauce mixture (and half of the fried peppercorns if desired), and let simmer for a while, or until sauce is reduced to the consistency of a thin glaze. Add fried chicken pieces, fried chillies, toasted cashew nuts (or peanuts), and toss vigorously to evenly coat the chicken with sauce. Scoop out onto a serving dish, garnish with chopped spring onions, and serve immediately. Enjoy hot, with plain steamed rice.