In a medium bowl, whisk together soy sauce, brown sugar, minced garlic, grated ginger, sesame oil, chopped green onions, black pepper, and optional chili flakes until the sugar dissolves. Set aside.

Slice boneless chicken thighs or breasts into thin strips, about ¼ inch thick. Add the chicken strips to the marinade, tossing to coat thoroughly. Let sit for at least 20 minutes.

Heat a lightly oiled skillet over medium-high heat. Once shimmering, add the marinated chicken strips in a single layer, avoiding overcrowding.

Cook the chicken for about 5-7 minutes, turning occasionally until no longer pink in the center and caramelized.

Transfer the cooked chicken to a serving plate. Sprinkle with toasted sesame seeds and extra sliced green onions. Serve hot over rice or with fresh vegetables.