Cut each chicken breast in half lengthwise to make 4 fillets. Cover with plastic wrap and, using a meat mallet or rolling pin, pound until chicken is about ½ inch thick. Season both sides with salt and pepper. Place 2 slices of prosciutto di Parma on top of each chicken piece. Top each with 2 sage leaves and weave a toothpick through to secure them in place. In a skillet, heat the oil over medium heat. Cook the chicken for about 2 minutes on each side, until cooked through. Transfer to serving plates. Add the marsala or white wine to the oil in the pan, and reduce over high heat for 2 minutes. Add the butter. Return the saltimbocca and any juices to the pan, turning them in the sauce for about 1 minute. Remove the pan from the heat and remove the toothpicks from the saltimbocca. Serve with juices drizzled over.