1/4 cup water
2 tablespoons packed dark brown sugar
4 teaspoons soy sauce or tamari
1 tablespoon rice vinegar
1 1/2 teaspoons toasted sesame oil
1/3 cup plus 1 teaspoon cornstarch, divided
1 1/2 pounds boneless, skinless chicken thighs or breasts, cut into 1-inch pieces
1 1/2 teaspoons kosher salt
3 tablespoons vegetable oil
2 cloves garlic, minced
2 tablespoons toasted white sesame seeds
1 medium scallion, thinly sliced
Cooked rice, for serving (optional)