Place breasts on foil and dot with 1 oz of butter. Season with salt and pepper and wrap well. Bake at 400F/200C for about 15-20 minutes.
Meanwhile make the sauce by melting remaining butter in a bowl sitting in boiling water. Stir in the mustard and then the vinegars. Cook for 8 minutes and then add the cream and season with salt and pepper.
Place breasts in a warm serving dish and pour over the sauce. Sprinkle with cayenne pepper. Serve with rice or couscous maybe.