Add reserved water a bit at a time, until the pasta is coated in a thin sauce (you may not need all the reserved water).
Stir in parmesan cheese.
Serve, garnished with additional parmesan cheese and chopped chives or parsley.
Notes
Make-Ahead: You can prepare the shallots and mushrooms in advance, and of course the black garlic butter, and keep them in the refrigerator. But we’d recommend making the pasta and tossing it with the sauce right before you’re ready to serve. Storage: If you have leftovers and want to reheat this dish, you might find the pasta is a little dry, since it will have absorbed some of the moisture from the sauce. Add in a bit more black garlic butter and a splash of water before reheating. That should coat the pasta in the sauce again.