Heat oven to 375°F. Butter 1 1/2-quart casserole or soufflé dish.
Melt 1/4 cup butter in medium saucepan.
Stir in flour, mustard and cayenne.
Cook until bubbly.
Stir in milk.
Cook and stir until mixture boils.
Cook and stir 1 minute.
Stir in cheese until melted; set aside.
In large bowl, beat egg whites and cream of tartar with whisk on speed #10 until stiff peaks form; set aside.
In medium bowl, beat egg yolks with whisk on Speed #8 for 3 minutes or until thick and light lemon-color. Switch to beaters and beat on Speed #3, adding cheese sauce, until mixed. Fold mixture into egg whites.
Pour into casserole. Bake 25 to 30 minutes or until deep golden brown and set.