Place the boneless chicken breasts side by side on a cutting board, and cover them with a piece of plastic wrap. Pound them with a flat meat hammer until they are 1/4 inch thick.
Season the all-purpose flour with a bit of salt and black pepper. Mix well with a fork or wooden spoon.
Heat the oil in a medium-size skillet over medium heat until hot. When the oil is hot, dip the chicken pieces into the seasoned flour and then shake off any excess.
Cook the cutlets for 5 minutes on each side, turning them once until golden brown. If the pieces don't fit comfortably into the pan, cook them in batches.
Sauté the prosciutto in the pan drippings for 1 minute to release some fat, then lower the heat to medium.
Sauté the mushrooms for 5 minutes until they are golden brown and most of the moisture has evaporated. Season with salt and pepper.
Pour the sweet Marsala wine into a medium-size skillet and heat it for a few seconds to cook out the alcohol content.
Add the chicken stock and simmer for a minute or until the sauce has slightly reduced.
Add the butter until melted to the pan and return the chicken cutlets. Make sure the sauce is spread over chicken pieces. Simmer gently for 1 minute to warm it through.
Before serving, sprinkle with chopped fresh parsley and a pinch of salt, and a teaspoon pepper. ENJOY!