Place chicken breasts between 2 sheets of heavy plastic on a solid, level surface. Firmly pound chicken with the smooth side of a meat mallet to a 3/8-inch thickness. Season lightly with salt and freshly ground black pepper.

With the smoother side of chicken breasts facing up, lay 2 to 4 sage leaves on each breast; more sage leaves will give a stronger sage flavor. Then, cover breasts with a single layer of prosciutto, covering the sage underneath.

Lay a piece of plastic wrap on top, and press down firmly to flatten. Refrigerate for about 30 minutes before cooking for best results.

Heat olive oil in a nonstick skillet over medium-high heat. Place chicken, prosciutto-side down, in the pan, and sear until prosciutto is lightly browned, about 3 minutes. Turn chicken over, and cook until chicken is no longer pink at the center and juices run clear, about 3 minutes more. Exact time will depend on size and thickness of chicken breasts.

Turn off heat, remove chicken to a plate, and cover loosely with foil.

For the sauce, add Marsala, lemon juice, and chicken broth to the pan, along with any accumulated juices from the plate with chicken, and place back over medium-high heat. Boil until reduced by half, about 3 minutes. Reduce heat to low, and swirl in butter until it disappears.