In a large bowl, combine the soy sauce, brown sugar, cornstarch, and chicken broth. Whisk until smooth and set aside.
Heat the vegetable oil and sesame oil in a large skillet or wok over medium-high heat.
Add the chicken pieces to the skillet and season with salt and pepper. Cook for about 5-7 minutes, or until the chicken is browned and cooked through.
Add the minced garlic, ginger, and the white parts of the green onions to the skillet. Stir-fry for 1-2 minutes until fragrant.
Pour the sauce mixture over the chicken and bring to a simmer. Cook for an additional 3-4 minutes, stirring frequently, until the sauce thickens.
Stir in the red pepper flakes if using, and the green parts of the green onions. Cook for another minute.
Serve hot over cooked rice or noodles.