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, turning occasionally, about 6-8 minutes. Drain on paper towels to remove excess grease.
03 - Lightly butter bread slices if desired and toast in a toaster or on a grill pan until golden brown and crisp.
04 - Lay out 3 slices of toasted bread for each sandwich. Spread mayonnaise (and Dijon mustard if using) on one side of each slice.
05 - On the first slice, arrange romaine lettuce leaves, tomato slices, and half of the seasoned sliced chicken.
06 - Place a second bread slice on top, mayonnaise side down. Layer additional lettuce, tomatoes, 2 slices of crispy bacon, and remaining chicken.
07 - Top with the third bread slice, mayonnaise side down. Press gently to settle layers.
08 - Insert toothpicks through opposite corners of each sandwich to hold layers together during slicing.
09 - Using a sharp knife, slice each sandwich diagonally into halves or quarters. Repeat assembly process for remaining sandwiches. Serve immediately while bread remains crisp.