Start heating oven to 300F. In saucepan heat, but do not scald, milk. Separate eggs, putting whites in large bowl, yolks in smaller one.
In double boiler melt butter or margarine. Stir in flour, then heated milk, salt and cayenne; cook, stirring until smooth and thickened. Thinly slice cheese right into sauce. Stir until cheese melts completely and sauce is velvety smooth; remove from heat.
With fork, beat egg yolks until well blended. Stir in a little cheese sauce. Slowly stir this mixture back into rest of cheese sauce. With electric mixer or hand beater, beat egg whites until stiff but not dry. Slowly pour in cheese sauce, folding until no large areas of egg white remain.
Pour mixture into ungreased 2-quart casserole up to within 1/4-inch of top. (Bake any extra mixture in small ungreased casserole).
To form crown, with teaspoon, make shallow path in soufflé mixture about 1-inch from edge of casserole all the way around. Bake, uncovered, 1-1/4 hour; don't open oven while soufflé is baking.
Serve at once. A mushroom sauce, sauteed tomato halves and crisp bacon are nice accompaniments.