In a large saucepan, bring the remaining 8 tablespoons of butter, the milk, cayenne and 2 1/2 teaspoons of salt to a boil. Remove from the heat and add the flour, stirring vigorously with a wooden spoon until combined. Cook the dough over moderate heat, stirring constantly, until it comes away from the sides of the pan, about 2 minutes. Remove from the heat. Using a handheld electric mixer, beat in the eggs, one at a time, beating well between additions, until a soft, silky dough forms. Stir in the shiitakes, Gruyère, Parmesan and parsley.