In a small saucepan, bring the cream to a simmer over low heat. Stir in the butter until melted, then whisk in the Parmesan until smooth. Add the Fontina and cook, whisking, until the cheese is melted and the sauce is smooth, about 3 minutes. Remove from the heat, stir in the grappa and season the sauce lightly with salt. Serve at once.
Make Ahead: The sauce can be kept at room temperature for up to 6 hours. Whisk over low heat to rewarm.
Serve With: Twice-baked potatoes (small ones are best); cooked mild greens, such as spinach or Swiss chard; fresh fettuccine or pappardelle; broccoli; soft or fried polenta; baked tomatoes; braised endive