Since you are pairing the cheese with something with high salt content, you need one that is MILD and can counteract the saltiness, not overpower the prosciutto, and add another dimension (texture, depth, vehicle for salt and cure).
I agree with the following suggestions: provolone (domestic, not aged) and Fontina Val D'Aosta. Domestic Mozzarella (and even some lovely Mozz di Bufala, if you want to spend that kind of dough) or a very young Gouda or Edam will also work. Parm and domestic Asiago both tend to be salty, which will not provide a counterpoint to the prosciutto. Parm or Domestic Asiago would be nice on top of the chicken, grated, and browned just before you serve it. Any young, mild, cheese with a firm texture will work. Aged cheeses tend to have a higher salt content, as a cheese that has lost water will become naturally more concentrated in salts.
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