Cut the chicken into 1-inch pieces and put them in a mixing bowl. In a small bowl, whisk together the egg, 2 tablespoons of the cornstarch, and a little salt and pepper. Add the egg mixture to the chicken and toss well to coat.

In another bowl, stir together the soy sauce, rice vinegar, sugar, water, sesame oil, ginger, garlic, remaining cornstarch, and half the sesame seeds. Set aside.

Heat a large skillet with 2 tablespoons of vegetable oil over medium-high heat. When the oil is hot, add the chicken and all the egg mixture. Cook, stirring occasionally, until golden brown and cooked through, 7 to 10 minutes.

Pour the sauce over the chicken in the skillet and briskly stir or toss the chicken to coat it with the sauce, which will begin to thicken as soon as it hits the hot skillet. Once the chicken is coated, turn off the heat.

Serve next to a bed of steamed white rice or cooked snow peas. Top with the remaining sesame seeds and chopped scallions.