Whisk ¼ cup soy sauce, ¼ cup packed light brown sugar, ¼ cup water, 1 tablespoon minced peeled ginger, 4 minced garlic cloves, 2 teaspoons cornstarch, and 2 teaspoons rice vinegar together in a small bowl until the cornstarch is suspended.
Pat 2 pounds boneless, skinless chicken thighs dry with paper towels. Season all over with ½ teaspoon kosher salt and ½ teaspoon black pepper. Place ⅓ cup cornstarch in a shallow dish. Coat each piece of chicken all over in the cornstarch, shaking off the excess.
Heat 3 tablespoons olive oil in a 12-inch skillet over medium-high heat until shimmering. Add the chicken in a single layer and sear until browned and cooked through, 7 to 9 minutes per side.
Reduce the heat to medium. Stir the sauce again and pour into the pan. Cook, stirring often and flipping the chicken occasionally, until the sauce thickens, about 2 minutes. Garnish with thinly sliced scallion greens and serve with steamed white rice.
