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 well combined.

Using a sharp knife, slice chicken breast into thin strips, about 1/4 inch thick.

Pour the marinade over the sliced chicken, ensuring each piece is well coated. Cover and let it marinate at room temperature for at least 20 minutes.

Heat sesame oil in a large skillet over medium-high heat until shimmering.

Add the marinated chicken in a single layer. Cook for about 5-7 minutes, turning occasionally until the chicken is browned and cooked through.

Transfer the cooked chicken bulgogi to a serving plate, garnishing with toasted sesame seeds and extra chopped green onions. Serve warm alongside cooked rice or lettuce wraps.