Mix ricotta, spinach, and egg yolk until combined. Add just enough flour to bind the mixture. Flour your hands, and roll about 1 tbs of the dough into a small ball. If the mixture sticks to your hands, add a small portion of additional flour. Add the gnocchi to a pot of salted water at a rolling boil, and boil them until they float. Remove and drain. Serve topped with melted butter and fresh grated parmesan.