In a medium bowl, combine soy sauce, brown sugar, minced garlic, grated ginger, sesame oil, chopped green onions, black pepper, and optional chili flakes. Whisk until the brown sugar is fully dissolved.

Toss the chicken pieces in the marinade, ensuring each piece is well-coated. Let the chicken marinate at room temperature for at least 20 minutes.

Heat a lightly oiled skillet over medium-high heat. Place the marinated chicken in a single layer and cook for about 5-7 minutes, turning occasionally until cooked through.

Once cooked, transfer the chicken to a serving plate and garnish with toasted sesame seeds and additional chopped green onions.