In a medium bowl, combine soy sauce, brown sugar, minced garlic, and grated ginger. Add sesame oil, green onions, black pepper, and optional chili flakes. Whisk until smooth.

Slice chicken into thin strips and combine with the marinade in a large bowl or resealable plastic bag. Marinate for at least 20 minutes.

Heat skillet over medium-high heat, add marinated chicken in a single layer. Cook for 5-7 minutes, turning occasionally, until fully cooked and caramelized.

Transfer cooked chicken to a serving plate. Sprinkle with sesame seeds and additional green onions, serve warm.