In a microwave safe dish add the butter, cream cheese and cheddar. On half power heat and stir alternatively until mixture is melted. Whip with a spoon to make sure it is well blended. Add cayenne, mustard, salt and pepper and stir to combine. Fold in the egg whites. Dunk bread cubes into the mixture and place on a wax paper lined sided cookie sheet. Freeze for 1 hour. Remove the cubes and place in a ziplock bag back in the freezer until ready to bake.
When ready to serve, preheat the oven to 375F. Place frozen cheese puffs on a baking sheet lined with parchment or a baking mat and bake for 10 min., watching that they don't burn. Serve hot or at room temperature.