In a small bowl, combine 1/2 cup water, peanut butter, soy sauce and brown sugar until smooth; set aside. In a skillet or wok, stir-fry garlic in oil for 30 seconds. Add the chicken; stir-fry for 5 minutes or until no longer pink. Add the broccoli; stir-fry for 5 minutes.
Stir in the peanut butter mixture; cook and stir for 2-3 minutes or until sauce is smooth and broccoli is crisp-tender. Combine cornstarch and remaining water until smooth; gradually add to skillet. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with rice or noodles.
Reduced-fat peanut butter is not recommended for this recipe.