Place your water on to boil for your pasta and remember to cook it while you are doing the rest of the recipe.
To flatten breast out lay them on a cutting board with plastic wrap over them and pound flat using what ever device you wish I use my hand, they need to be about a quarter inch thick. Season with salt and pepper.
Heat the oil until it is just before smoking. Pan fry until golden brown on both sides, about 5 minutes per side. Remember to not crowd the pan. Remove the chicken and set aside.
Lower the heat a little, add the mushrooms and saute until they are brown and have shrunk a lot and lost their moisture. Should be about 5 minutes, season with salt and pepper. Add marsala to the pan and bring to boil and let sit for about a minute. Add the chicken stock and simmer to reduce.
Stir in the butter. If you want it a little thicker mix some cornstarch with warm water and add to the sauce a little at a time stirring until desired thickness is obtained. Add chicken back in and let it warm back up.
Serve over the spaghetti with mushrooms and sauce on top. Garnish with parsley if you wish
Be careful reducing the sauce if you want to use it with pasta. I would recommend starting with a full cup of marsala wine and chicken stock.