In a medium saucepan combine the cheeses and butter or margarine.
Cook and stir over low heat till melted; remove from heat.
Beat egg whites till stiff peaks form (tips stand straight).
Fold cheese mixture into egg whites.
Dip bread cubes into cheese mixture, scraping off the excess with a spatula.
Place coated bread cubes on a baking sheet lined with waxed paper.
Freeze till firm (30 to 60 minutes).
Transfer to plastic bags; freeze.
At serving time, place frozen cubes on a lightly greased baking sheet.
Bake in a 400Â° oven about 10 minutes or till puffed and golden brown.