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

Slice chicken into thin strips and toss in the marinade, ensuring each piece is coated. Cover and let marinate for at least 20 minutes.

Heat a lightly oiled skillet over medium-high heat. Add marinated chicken in a single layer and cook for 5 to 7 minutes until fully cooked and caramelized.

Remove chicken from heat and transfer to a serving plate. Garnish with toasted sesame seeds and additional green onions. Serve warm.