In a small bowl, toss the chicken pieces with the cornstarch, making sure to get all the chicken coated.
Heat the oil in a wok or large sauté pan over medium-high heat. When the oil is hot, brown the chicken on all sides. Do this in batches so you don’t crowd the chicken, as the chicken needs space in the pan to brown well. Set the browned chicken aside on a plate.
Add the garlic, ginger and hot pepper to the pan and stir fry for a few minutes. Add most of the scallions (reserve a handful for garnish) and the chicken back into the pan and toss to combine.
Add the vinegar, hoisin sauce, soy sauce, brown sugar (if using) and the water to the pan. Mix everything well then bring up to a simmer and cook gently for a minute or two until the chicken is cooked through and the sauce is thick and glossy. Taste to adjust seasonings according to preference.
Add the cashews to the sauce and sprinkle on a few dashes of the sesame oil. Serve over hot rice garnished with the rest of the scallions, snipped chives and a few extra cashews.