In a mixing bowl, combine soy sauce, sriracha, minced garlic, ginger, toasted sesame oil, red chili flakes, salt, and pepper to create a flavorful marinade. Toss the boneless, skinless chicken thighs in the marinade, ensuring they are fully coated. Cover and refrigerate for at least 2 hours, or up to 8 hours for maximum flavor absorption.

About 15 minutes before cooking, preheat your grill to medium heat, aiming for a temperature between 375-400°F. Clean and oil the grill grates lightly to prevent sticking.

While the grill is heating up, mince the green onions and set aside. In a dry skillet over medium heat, toast sesame seeds for about 2-3 minutes until golden brown and fragrant.

Remove the marinated chicken thighs from the refrigerator and place them directly on the preheated grill. Cook for 10-12 minutes, flipping every 2-3 minutes. In the last 2 minutes of grilling, brush with Japanese BBQ sauce.

Check that the internal temperature of the chicken thighs has reached 165°F using a meat thermometer.

Once cooked, transfer the chicken thighs to a serving platter and garnish with toasted sesame seeds and minced green onions. Serve alongside extra BBQ sauce and yum yum sauce for dipping.