Cut the chicken into bite-sized chunks then put them into a bowl. Add cornstarch, salt, and water. Mix very well so that the chicken is well coated and becomes sticky.
Add 1 teaspoon of oil. Stir to distribute evenly. Leave to marinate for 10 minutes.
Sear the chicken
Heat an empty wok/skillet over high heat until smoking hot, then pour in 1 tablespoon of oil and swirl to coat a large area (If using cookware with a non-stick coating, add oil first then heat up).
Add the marinated chicken. Spread it into a single layer and leave it to sear. Once the bottom side turns pale, flip and stir to quickly sear other sides.
Dish out the chicken as soon as there is no more pinkness to be seen (It is not yet cooked inside at this stage).
Combine the dish
Add the remaining ½ tablespoon of oil to the wok/skillet. Put in scallions and chili pepper. Stir and toss over high heat for about 30 seconds, or until the scallions start to wilt.
Put the chicken back in the wok, then add black bean sauce. Stir fry for a further one minute until the chicken is fully cooked.
Dish out and serve immediately with steamed rice, alone or with other savory dishes.
Notes
Shop-bought black bean sauce is usually much saltier than the homemade version that I use for this recipe. I suggest you add 1 tablespoon first, then taste and adjust as needed.