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 completely dissolves and the marinade is well-blended.

Slice the chicken into thin, even strips and add to the marinade, tossing to coat. Let it sit for at least 20 minutes at room temperature or refrigerate for up to 2 hours.

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

Transfer cooked Chicken Bulgogi to a serving plate, garnish with toasted sesame seeds and additional green onions, serve warm with rice, kimchi, or lettuce wraps.