Use aged mizithra, not fresh, and grate it yourself from a block — pre-grated cheese is coated in starch that stops it melting. Ricotta salata is the closest substitute, or two parts ricotta salata to one part Pecorino Romano. Brown the butter in a light-colored pan so you can see the color; it goes from amber to burnt in under a minute. Serve immediately, as this dish does not hold or reheat well.