In a large bowl or resealable bag, combine soy sauce, sriracha, minced garlic, ginger, sesame oil, red chili flakes, salt, and pepper. Mix well to create a flavorful marinade. Add the boneless, skinless chicken thighs, ensuring they are evenly coated. Seal the bag or cover the bowl, then refrigerate for 2–8 hours to let the flavors infuse deeply into the chicken.

About 15 minutes before grilling, preheat your grill to medium heat, aiming for 375–400°F. This temperature will ensure that the chicken cooks through without burning the marinade. Use a grill thermometer for accuracy, and make sure the grates are clean and lightly oiled to prevent sticking.

While the grill is heating, prepare your garnishes by mincing fresh green onions and toasting sesame seeds. In a dry skillet over medium heat, add the sesame seeds and toast for 2–3 minutes, stirring frequently until they are golden and fragrant. Set them aside for final assembly.

Remove the marinated chicken thighs from the fridge, allowing them to come to room temperature for about 10 minutes. Place them on the preheated grill and cook for 10–12 minutes, flipping every 2–3 minutes for even cooking. In the last 2 minutes of grilling, generously brush on your Japanese BBQ sauce.

Use a meat thermometer to check the internal temperature of the chicken, ensuring it reaches 165°F for safe consumption. The exterior should appear beautifully browned and glossy with the BBQ sauce.

Once cooked, remove the chicken from the grill and transfer it to a platter. Sprinkle the toasted sesame seeds and minced green onions generously over the top for an appealing finish. Serve with yum yum sauce for dipping.