Melt cheeses and butter together in top of double boiler over hot water. Beat till smooth and completely blended. Cool to lukewarm. Fold in 2 stiffly beaten egg whites. Cut unsliced loaf into 9-10 slices (1/2" to 3/4" thick); cut slices into cubes. Dip cubes into cheese mixture to coat completely. Put on cookie sheet and freeze immediately. When frozen, store in plastic bags in freezer. (Will keep for weeks.) To serve, place frozen cubes on greased cookie sheet and bake at 450° for 8 minutes, or until golden brown. Serve hot.