Easy Mongolian Chicken

In a mixing bowl, toss the chicken pieces with cornstarch, garlic powder, salt, and black pepper until evenly coated.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the chicken pieces in batches and fry until golden brown and crispy, about 4-5 minutes per side. Remove and set aside on paper towels.
In the same skillet, add a little more oil if needed and sauté minced garlic and grated ginger for 1-2 minutes until fragrant.
Pour in soy sauce, water, brown sugar, sesame oil, and rice vinegar (if using). Stir to dissolve the sugar and bring to a simmer.
Add the cornstarch slurry and cook for 1-2 minutes until the sauce thickens and becomes glossy.
Return the crispy chicken to the skillet and toss to coat evenly in the sauce.
Add sliced green onions and red pepper flakes if using, cook for another 1-2 minutes until heated through.
Garnish with sesame seeds and extra green onions before serving.