In a large sealable bag, whisk together the balsamic vinegar, soy sauce, honey, olive oil, minced garlic, minced ginger, chopped scallions and salt. Add the chicken, seal the bag, and then carefully move the chicken around in the bag until it's thoroughly coated. Place the chicken in the fridge to marinate overnight or for a minimum of 2 hours.
When ready to bake, preheat the oven to 350ºF with the rack in the center. Line a baking sheet with foil.
Remove the chicken from the bag and reserve the marinade. Shake off any excess marinade from the chicken thighs and arrange them skin side down on the baking sheet. Bake the chicken for 30 minutes.
Remove the chicken from the oven and flip the thighs skin side up. Turn the oven to broil, and if necessary, adjust the baking rack so it is about 8 inches from the broiler. Return the chicken to the oven and broil for about 5 minutes until the skin is crisped and the chicken reaches an internal temperature of at least 165ºF. (The chicken juices should run clear, rather than pink.) While the chicken is cooking, transfer the marinade to a medium saucepan.
Whisk together the cornstarch with 2 tablespoons water. Bring the marinade to a boil then whisk in the cornstarch mixture and continue boiling the marinade, whisking intermittently, until it thickens to the consistency of syrup, about 5 minutes. (The glaze must come to a boil to get rid of any potential bacteria from the raw chicken.) Remove the glaze from the stove and set it aside.
Remove the cooked chicken from the oven, brush it with the prepared glaze and serve immediately.