In a bowl, combine the chicken pieces, soy sauce, and cornstarch. Mix well and let marinate for at least 15 minutes.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken and cook until browned and cooked through, about 5-7 minutes. Remove the chicken from the skillet and set aside.
In the same skillet, add the white parts of the scallions, garlic, and ginger. Stir-fry for about 1-2 minutes until fragrant.
Return the chicken to the skillet and add the sesame oil, rice vinegar, and sugar. Toss everything together and cook for an additional 2-3 minutes.
Season with salt and pepper to taste and stir in the green parts of the scallions just before serving.