In a medium saucepan, bring the stock to a simmer and crumble in the saffron. Maintain at a simmer over moderately low heat. In a large nonreactive saucepan or flameproof casserole, heat the oil over moderate heat. Add onion and prosciutto and cook until the onion is softened and translucent, about 2 minutes. Add the rice and stir for 1-2 minutes until well coated with oil and slightly translucent. Add the Marsala and cook, stirring until it evaporates. Add 1/2 cup of the simmering stock and cook, stirring constantly, until the rice is almost tender but still slightly crunchy in the center, 20-25 minutes. Add cheese and season with the pepper and salt to taste. Continue to cook, stirring and adding stock as necessary, 1/4 cup at a time, until the rice is tender but still firm and is bound with a creamy sauce 3-6 minutes longer. Stir in butter and serve hot.