In a bowl, whisk together soy sauce, water, sesame oil, brown sugar, rice vinegar, ginger, garlic and sesame seeds. Set aside.
In a large bowl, whisk together egg, cornstarch, salt and pepper. Toss the cubed chicken in the cornstarch mixture.
Heat vegetable oil in the skillet over medium-high heat. When oil is very hot (you can tell when it looks wavy on the surface or drip of water splatters) add the chicken with all of the egg mixture. Cook chicken, stirring occasionally, until golden brown and cooked through, 7-10 minutes.
Once cooked through, pour sauce over top tossing chicken to coat. The sauce will thicken and you can turn off the heat!
Serve with rice. Garnish with green onions and additional sesame seeds.