01 - Season the cooked chicken breast slices evenly with salt and black pepper on both sides.
02 - Place bacon slices in a skillet over medium heat. Cook until crispy, approximately 5-7 minutes. Transfer to paper towels to drain excess grease.
03 - Lightly butter bread slices if desired. Toast in a toaster or grill pan until golden brown and crisp.
04 - Arrange 3 slices of toasted bread per sandwich. Spread mayonnaise (and mustard, if using) on one side of each slice.
05 - On the first bread slice, layer romaine lettuce leaves, tomato slices, and half of the seasoned chicken slices.
06 - Place second bread slice mayonnaise-side down. Add additional lettuce, tomato slices, and 2 crispy bacon slices.
07 - Top with third bread slice, mayonnaise-side down. Press gently to settle layers.
08 - Insert toothpicks at opposite corners to hold the triple-decker structure together.
09 - Cut each sandwich diagonally into halves or quarters. Serve immediately while bread remains crispy.