1. Gently warm the milk in a saucepan and remove from the heat just before it boils.
2. Cut the prosciutto into small pieces.
3. Melt the butter in a sauce pan and add the prosciutto.
4. Let it sauté for one minute and then add the wheat and corn flour. Stir well while the mix thickens and continue to stir until it begins to smell slightly nutty. Be careful that it does not become too dark.
5. Add the hot milk a little at a time while stirring.
6. Remove the dough from the heat and add chunks of cheese.
7. Whip the dough thoroughly with either an electric whisk or a mixer until the consistency is soft and fluffy and the cheese has melted.
8. Let the dough cool in the refrigerator for at least four hours.
9. Place the dried porcini mushrooms in a blender and blend until the mushrooms have a consistency like breadcrumbs.
10. Put the extra flour for coating in a bowl. Beat the eggs together in a second bowl. Mix breadcrumbs and blended porcini mushrooms in a third.
11. Form small balls of dough (a few centimetres in diameter). Roll them in flour, then in the beaten egg and finally in breadcrumbs and porcini mushrooms. Be sure the breadcrumbs coat the ball completely.
12. Heat the frying oil in a saucepan. When the oil sizzles, it is warm enough. Fry the croquettas until they are golden and crisp. Only fry a couple at a time to avoid them sticking together and to keep the oil from cooling down.
13. Remove the fried balls and place them on fat absorbent paper. Let them cool down a bit - they taste the best when they are not burning hot. Remove the croquettas and place them on baking paper to absorb the excess frying oil. Let them cool down slightly before eating.
14. Serve the croquettas with a good, thick balsamic or sherry vinegar and thin slices of prosciutto.