Mix together cornstarch, salt, pepper, and Chinese five-spice powder in a large bowl. Cut the chicken thighs into bite-sized pieces.
Heat the oil in a pan, wok, or skillet (Note 2).
When the oil is hot, place ½ of the chicken pieces into the bowl with the cornstarch and spices and toss until coated.
Immediately and carefully place them into the hot oil and fry until golden brown, about 2-3 minutes, moving the pieces a few times in between. Check if the meat is cooked through by cutting a piece in the middle. Remove with a slotted spoon and place on kitchen paper towels to remove the excess fat.
Repeat with the second batch, only coating the chicken pieces with cornstarch just before adding to the frying pan.
Pour away all the fat from the pan, keeping only about 2 tablespoons of it. Add the vegetables and stir fry for about 2 minutes. Add salt and pepper, mix well.
Mix with the chicken pieces and serve.