1. Preheat the oven to 375°. Butter a 1 1/2-quart soufflé dish and
coat it with 2 tablespoons of the Parmigiano.
2. In a medium saucepan, melt the butter. Stir in the flour to
make a paste. Gradually whisk in the cream and bring to a boil
over moderate heat, whisking. Reduce the heat to low and cook,
whisking, until very thick, 3 minutes. Transfer the base to a large
bowl; let cool. Stir in the egg yolks, sherry, Gruyère, sour cream,
salt, Dijon mustard, dry mustard, cayenne and the remaining
1/4 cup of Parmigiano.
3. Put the 7 egg whites in a large stainless steel bowl. Add the
cream of tartar. Using an electric mixer, beat the whites until firm
peaks form. Fold one-third of the whites into the souffl. base to
lighten it, then fold in the remaining whites until no streaks
4. Scrape the mixture into the prepared dish. Run your thumb
around the inside rim of the dish to wipe away any crumbs. Bake
for about 35 minutes, until the souffl. is golden brown and puffed.
Serve right away.