Cut the chicken into ½ inch strips and cut the strips into ½ inch cubes. Place them in a bowl with the marinade ingredients. Mix and set aside. In a separate bowl, combine the sauce ingredients.
Do your prep: Peel and thinly slice (do not chop or mince) the garlic and ginger. Slice the white parts of the scallions into chunks about as long as they are wide, so that they have a cube shape to match the chicken. Thinly slice some of the green parts and set aside to garnish the plate later. Snip the chilies in half with a pair of scissors and remove the seeds, using gloves or an appropriate kitchen tool. Measure out the peanuts and Sichuan pepper. Set everything next to the stove and prepare a serving dish - things will go quickly once the flame goes on.
Heat a well-seasoned wok over high heat and add the peanut oil. When it is hot but not yet smoking, add the chilies and Sichuan peppercorns and fry for 30-60 seconds until they are fragrant.
Add the chicken, followed by the garlic, ginger, and scallions. Stir-fry until the chicken is opaque and just cooked through. This should take about five minutes, but test by cutting through one of the thicker pieces.
Give the sauce ingredients a good stir and add them to the wok, tossing the sauce with the chicken. When the sauce has thickened, add the peanuts. Give everything a good stir and remove to a serving dish.
Top the dish with the thinly-sliced scallions and serve with some steamed rice, a cold beer, and a box of tissues.
Variation: For a flexetarian version, reduce the chicken to ½ pound and add 7 ounces (half a package) of firm tofu. Drain it and dry it well with paper towels, then cut it into ½-inch cubes to match the chicken. Do not marinate the tofu. Instead, add it to the pan about 3 minutes after the chicken goes in, before the sauce.