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 sugar is fully dissolved.

Slice the chicken into thin strips and toss into the marinade, ensuring each piece is well-coated. Let marinate for at least 20 minutes at room temperature or up to 2 hours in the refrigerator.

Heat a lightly oiled skillet over medium-high heat. Add the marinated chicken in a single layer and cook for about 5-7 minutes, stirring occasionally, until fully cooked and golden brown.

Transfer cooked bulgogi to a serving plate and sprinkle with toasted sesame seeds and additional chopped green onions. Serve warm with steamed rice or your favorite sides.