Broil peppers on an aluminum foil-lined baking sheet 5 inches from heat about 5 minutes on each side or until peppers look blistered.
Place broiled peppers in a heavy-duty zip-top plastic bag; seal bag and let peppers stand about 10 minutes to loosen skins. Peel peppers; remove and discard seeds.
Process peppers, cooked tomatillos, 1 cup chicken broth, and next 5 ingredients in a food processor or blender until mixture is smooth, stopping to scrape down sides. Transfer mixture to a saucepan, and cook over medium heat 5 minutes. Slowly stir in remaining 1/2 cup chicken broth, and cook about 10 minutes or until sauce is thickened.