Prepare the Marinade: Whisk together balsamic vinegar, olive oil, Dijon mustard, garlic powder, and seasonings (add soy sauce, sugar, or herbs if desired). Set aside.
Marinate the Chicken: Place chicken in a resealable bag or shallow dish and pour marinade over it. Refrigerate for at least 30 minutes.
Cook the Chicken:
Preheat the oven to 400°F (200°C).
For breasts: Bake for 20 minutes, or until the internal temperature reaches 165°F (74°C).
For thighs: Bake for 25 minutes, checking internal temperature.
Create the Glaze: Simmer the remaining marinade over low heat, stirring constantly, until it reduces to a thick syrup-like consistency.
Serve: Brush the reduced glaze over the chicken and serve.