In a medium saucepan, combine the soy sauce, sugar, pineapple juice, garlic powder, ginger powder, and black pepper. Bring to a boil over medium heat, stirring occasionally. When the mixture begins to foam/rise, immediately remove from the heat.

Remove 1 cup of the mixture from the saucepan. Set it aside and let it cool to room temperature. This will be used as a marinade.

In a small bowl, whisk together the cornstarch and water to create a slurry.

Place the saucepan with the remaining sauce over medium-high heat. Slowly whisk the cornstarch slurry into the sauce. Continue stirring until the sauce thickens to your desired consistency, about 1 to 2 minutes. Remove from the heat and let cool completely before transferring to an airtight container to refrigerate. The sauce will keep in the refrigerator for up to 2 weeks.

Place the chicken thighs in a large ziptop bag or shallow glass dish and pour the reserved marinade over the chicken thighs. Seal the bag (pressing out excess air) or cover the dish tightly. Refrigerate for at least 2 hours, or ideally 4 to 8 hours for maximum flavor development. Flip or redistribute the chicken in the marinade halfway through the marinating time.

Remove the chicken from the refrigerator about 30 minutes prior to cooking.

Preheat the grill to medium-high heat (375°F to 400°F) and clean the grates thoroughly. When the grill is hot, oil the grates well using tongs and a paper towel dipped in high-heat oil.

Remove the chicken from the marinade, allowing the excess to drip off. Discard the used marinade.

Place the chicken on the grill for 4 to 5 minutes with the lid closed until grill marks develop. Flip the chicken and continue grilling with the lid closed for another 4 to 5 minutes, until the internal temperature reaches 165°F at the thickest part. During the final 2 minutes of cooking, brush the reserved thickened teriyaki sauce onto both sides of the chicken.

Transfer the grilled chicken to a clean plate and tent loosely with foil. Let rest for 5 minutes to allow the juices to redistribute.

Sprinkle with toasted sesame seeds and sliced green onions and serve with additional teriyaki sauce on the side.