In a bowl, combine soy sauce, hoisin sauce, brown sugar, minced garlic, and minced ginger. Stir until the sugar is dissolved and set aside.
Heat vegetable oil in a large skillet over medium-high heat. Add the chicken pieces and season with salt and pepper. Cook until browned and cooked through, about 5-7 minutes.
Pour the sauce mixture over the cooked chicken in the skillet. Stir to coat the chicken evenly.
Reduce heat to medium-low and let the sauce simmer for about 5 minutes, allowing it to thicken slightly.
Stir in the chopped green onions and red pepper flakes (if using) and cook for an additional 2 minutes.
Serve hot over rice or noodles.