The success of this Roman dish depends on first-rate ingredients. If you can find an unsalted European-style butter, such as Plugra (widely available at gourmet groceries), use it here. It has a richer flavor and a lower water content than everyday brands. For the best taste, seek out heavy cream that is not ultrapasteurized. When you buy a chunk of Parmigiano-Reggiano, ask the clerk to cut a wedge fresh from the wheel to make sure it is not too dried out. In Italy, this luxurious dish is served in small portions as a first course. Try it before a simple main dish of poached or baked fish or grilled meat.