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, turning occasionally. 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 slice, place romaine lettuce leaves, followed by tomato slices, and half of the seasoned chicken slices.
06 - Top with a second slice of bread, mayonnaise side down. Add another layer of lettuce, tomato slices, and 2 crispy bacon pieces.
07 - Place the third bread slice, mayonnaise side down, on top to finish the triple-decker construction.
08 - Insert toothpicks through opposite corners of each sandwich to hold layers together during serving.
09 - Cut each sandwich diagonally into halves or quarters. Serve immediately while bread remains crisp.
10 - Follow the same assembly steps for the remaining 3 sandwiches.