To the same skillet, add the butter. Sauté the mushrooms and the shallots, stirring occasionally, for about 3 to 4 minutes or until golden brown. Add the Marsala wine and simmer for 1 to 2 minutes, scraping up the brown bits from the bottom of the pan. Add the chicken stock and return the chicken breasts to the skillet. Lower the heat to medium and continue to simmer until chicken is cooked through, about 2 to 3 minutes. Chicken is done when it feels firm to the touch and a meat thermometer inserted in the center reaches 170°F. Season sauce with remaining salt and pepper.