Place a veal cutlet on a cutting board and cover with plastic wrap. Use a kitchen hammer to pound the veal until 1/8" in thickness.
Set veal on a plate. Pound the other veal cutlet in this fashion. Season the veal with pepper (the prosciutto will add a salty taste).
Set a piece of prosciutto and 2 sage leaves on each piece of veal. Use tooth picks to secure them in place.
Place the butter and olive oil in a very large skillet set over medium-high. When the butter is melted and no longer foaming, set veal in the skillet, prosciutto-side down, and fry until golden brown on that side, about 2 minutes. Turn veal over and cook 2 more minutes.
Remove veal from skillet and set on a plate. Carefully remove toothpicks from veal.
To make sauce, place the Marsala and stock or broth in the skillet, bring to a simmer, and cook until reduced by a third. Set veal back in skillet, prosciutto-side up, and heat through 1 minute. Set a piece of the veal on each of 2 plates, spoon over the sauce and serve.
Note: Marsala is a sweet, fortified Italian wine available at most liquor stores. If you can’t find it, try sherry in this recipe, or, to make a less sweet sauce, use dry white or red wine.