In saucepan, combine soy sauce and brown sugar, vinegar and broth, boiling mixture until reduced by half. In skillet, heat oil over moderately high heat until hot; sauté chicken until brown. Transfer to plate. In remaining oil, cook garlic and onion with bay leaf and pepper, stirring until onion is soft. Add broth mixture and chicken with any juice on plate. Simmer the mixture, turning chicken until cooked thoroughly. Divide chicken between 2 plates. Stir the butter into the sauce in skillet. Cook, stirring constantly, until thickened slightly. Discard bay leaf. Season sauce with salt and pepper. Pour sauce over chicken on plates.