In a bowl, evenly coat the chicken with cornstarch seasoned with salt and pepper and set aside. (dont set the chicken pieces on top of one another). Toss the chicken in cornstarch one more time right before frying.
In a deep saucepan add the canola oil and heat it until the oil is hot. Start adding the chicken carefully and fry them until they cook (about 5 minutes per side, depending on the size of pieces to an internal temperature of 160 degrees using a thermometer). Do not overcrowd the pan. Fry in batches if needed.
Take out the chicken and place them onto a paper a towel lined plate.
In a separate saucepan, add in the ketchup, brown sugar, rice wine vinegar, and soy sauce. Heat the sauce over low to medium heat and stir well. Once it starts to thicken, remove the pan from the heat.
Spoon the sauce over the chicken, garnish with scallions and serve.