Balsamic BBQ Sauce

(from Giada de Laurentiis at foodnetwork.com)
1 c balsamic vinegar3/4 c ketchup1/3 c brown sugar1 garlic clove, minced1 T Worcestershire sauce1 T Dijon mustard1/2 t salt1/2 t pepper

Combine all ingredients in a small saucepan and stir until the mixture is smooth. Simmer over medium heat 15 to 20 minutes, or until the sauce has reduced by about a third. Brush the sauce on the chicken, then keep it warm while you brush the chicken frequently until the chicken is done. Pass the leftover warm sauce with the chicken.