Step 1

Heat oven to 375ºF.

Step 2

Melt butter in small saucepan on low heat. Stir in flour. Cook 2 min. until bubbly. Gradually stir in milk with whisk until smooth. Stirring constantly, bring to boil on medium heat; boil 1 min. Remove from heat. Beat the 4 egg yolks slightly in small bowl. Stir small amount of hot mixture into yolks. Stirring constantly, slowly pour egg mixture back into hot mixture. Stirring constantly, cook on low heat 1 to 2 min. or until thickened. Add cheeses; stir until melted. Set aside.

Step 3

Beat the 6 egg whites and cream of tartar in large metal or glass bowl with mixer on medium speed until stiff but not dry. Fold about 1 cup egg whites into the sauce with rubber spatula to lighten sauce. Gently fold the sauce back into the remaining egg whites. Carefully pour the soufflé mixture into ungreased 2-qt. soufflé dish or baking dish.

Step 4

Bake 20 min. or until puffed and set.