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 half 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, quarter 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.