Pound chicken breasts slightly to flatten. Place a piece of prosciutto and a slice of cheese on the underside of each breast and roll up, skewering with a toothpick. Dip chicken rolls in flour to coat, shake off excess, then dip in beaten egg, drain briefly and roll in mixture of bread crumbs, Parmesan cheese, garlic, salt and tarragon. Brown chicken rolls in butter in a large frying pan, turning to brown all sides. Transfer to a baking dish and pour in broth and sherry. Bake, uncovered, in a preheated 350° oven for 30 to 35 minutes. (Meat should be white at center.) Gently lift chicken and place on warmed serving platter. Drain juices from baking dish into a small saucepan. Bring quickly to a boil and blend in paste of cornstarch and water. Cook until thickened, stirring constantly. Spoon sauce over chicken and serve.