1Heat oven to 350°F. Butter 1-quart soufflé dish or casserole. Make a 4-inch-wide band of triple-thickness foil 2 inches longer than circumference of dish. Butter one side of foil. Secure foil band, buttered side in, around top edge of dish.
2In 2-quart saucepan, melt 1/4 cup butter over medium heat. Stir in flour, salt, mustard and red pepper. Cook over medium heat, stirring constantly, until smooth and bubbly; remove from heat. Stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in cheese until melted; remove from heat.
3In medium bowl, beat egg whites and cream of tartar with electric mixer on high speed until stiff but not dry; set aside. In small bowl, beat egg yolks on high speed about 3 minutes or until very thick and lemon colored; stir into cheese mixture. Stir about one-fourth of the egg whites into cheese mixture. Fold cheese mixture into remaining egg whites. Carefully pour into soufflé dish.
4Bake 50 to 60 minutes or until knife inserted halfway between center and edge comes out clean. Carefully remove foil band. Serve immediately by quickly dividing soufflé into sections with 2 forks.