Cut bread into one-inch cubes; set aside. Melt butter and cheeses in a double boiler over low heat, stirring often. Remove from heat; fold in egg whites.
Dip bread cubes into cheese mixtures; set on greased baking sheets. Place in freezer until frozen; remove from baking sheets and store in plastic zipping bags in the freezer up to one month.
To serve, bake frozen at 400° for 12 minutes on greased baking sheets. Serve with marinara sauce, if desired.