On hot grill char chilies until blackened on all sides. Wrap in paper bag and let stand 10 minutes to steam. Peel, seed and chop chiles. This step can be done ahead of time and can also be done in the broiler.
Combine chilies, cream, onion, cilantro, and garlic in large sauce pan over medium high heat. Boil this sauce until reduced to a thick sauce. This takes about five minutes. Be careful to stir to prevent burning.
Season chicken breasts with salt, pepper, and garlic powder.
Grill chicken until properly done being careful not to overcook.
Meanwhile add tomato to sauce and simmer. Season with salt and pepper to taste.
Spoon sauce on plates. Slice chicken diagonally into thin slices and place atop sauce.