Make the mash: boil the potatoes in salted water for 15-20 mins until tender.
Drain and mash well. Stir in the milk, then the Parmesan and basil. Season with black pepper and keep warm.
Meanwhile, sit the chicken breasts on a board and cut through them horizontally leaving one side attached. Open them up like a book and lay a basil leaf on top of each, topped with a slice of prosciutto. Secure with a cocktail stick.
Heat the oil in a large, non-stick frying pan and add the chicken breasts, hamside down. Cook for 2-3 mins until golden, then turn over and cook for 2 mins more.
Pour over the Marsala and allow to bubble for 1 min.
Add the stock and simmer for 5 mins until the chicken is cooked. Lift the chicken out, cover to keep warm, then boil the pan juices to reduce a little.
Serve with the basil mash and a green salad on the side, if you like.