Add teriyaki marinade ingredients to a medium saucepan and whisk very well. Be sure that all of the cornstarch is dissolved into the mixture.
½ cup soy sauce, ¼ cup mirin, 3 tablespoons brown sugar, ½ teaspoon grated fresh ginger root, 1 grated garlic clove, 1 & ½ teaspoons corn starch
Bring ingredients to a boil over medium high heat. Lower heat and simmer about 5-8 minutes until the sauce begins to thicken and coats a spoon.
Remove from heat and transfer to a large bowl, reserving 2-3 tablespoons for brushing the chicken right before serving. (You can also marinate chicken in a Ziploc bag. Just be sure teriyaki is fully cooled before pouring into a plastic bag).
Once sauce is fully cooled, add chicken thighs and use a spoon to submerge them fully. Cover, refrigerate and and marinate at least 20 minutes or up to overnight.
It's a good idea to add olive oil to your grill grates to prevent sticking. Add oil before you turn the grill on! Dab it onto a paper towel and brush onto the grates. You can also lightly spray grates with non-stick cooking spray but NEVER do this when the grill is on. It should be off because oil is very flammable and hazardous when sprayed near flames.
Next, turn the grill on and close the lid. Let it preheat to 400℉.
Arrange chicken on the grill and cook about 10 minutes per side. Use a meat thermometer to check internal temperature. Once it reaches 165℉ the chicken is done.
Turn the grill off, remove chicken and brush with reserved teriyaki. Allow it to rest for 10 minutes, then serve and enjoy!