First, prepare homemade teriyaki chicken marinade. Combine all of the ingredients in a medium bowl and whisk to comine. Reserve around 2 tablespoons of marinade to thicken for later.
Next, place 1.5-lbs. of chicken breast into a gallon-size plastic bag or large stasher bag. Then, add in the teriyaki marinade, making sure the chicken is fully covered. Let sit in the fridge for at least 30-minutes or up to 2 hours.
Preheat the grill to 425ºF and rub your grill grates with olive oil.
Grill chicken breast at 425ºF for 7-10 minutes on each side or until the chicken reaches an internal temperature of 160ºF - 165ºF. If you've got leftover marinade, we suggest using a meat brush to brush on some teriyaki marinade a few times during the grilling process to really infuse the chicken with flavor.
Remove chicken breast from grill and let rest for 5 minutes so that the chicken can continue cooking.
To thicken the remaining teriyaki sauce, pour the sauce you set aside into a small saucepan and bring to a boil. whisk 1 teaspoon of cornstarch with 2 teaspoons of water to create a cornstarch slurry and then add it to the pat. Whisk the sauce until thickened.
Serve grilled teriyaki chicken with the extra sauce and your favorite sides.