Place the chicken breasts on a cutting board, cover with wax paper and pound until they are very thin. Season with salt & pepper.
Place the flour in a bowl, season with salt & pepper.
Heat a nonstick pan and add the olive oil.
Lightly Dredge the chicken in flour, shake well to remove any excess and cook for about 5 minutes per side until done (cook in 2 batches if it won't fit). Remove the chicken and place on a cookie sheet with foil to keep it warm.
Add the prociutto and shallots to the pan and cook for 2-3 minutes until the prosciutto starts to crisp up.
Add the mushrooms and cook for 5 minutes.
Add the Marsala and cook for 3 minutes.
Add the chicken stock and cook for 3-5 minutes, until sauce has reduced. Taste for seasoning.
Add the parsley, butter and return the chicken with juices to the pan.
Recipe by Cook Eat Run at https://cookeatrun.com/2014/01/25/chicken-marsala/