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 5-7 minutes. Drain on paper towels to remove excess grease.
03 - Lightly butter bread slices if desired. Toast in a toaster or grill pan until golden brown and crisp.
04 - Lay out 3 slices of toasted bread per sandwich. Spread mayonnaise (and Dijon mustard if using) on one side of each slice.
05 - On the bottom slice, place romaine lettuce leaves, tomato slices, and half of the seasoned sliced chicken.
06 - Place a second bread slice, mayonnaise side down. Add another layer of lettuce, tomato slices, and 2 crispy bacon slices.
07 - Top with the third bread slice, mayonnaise side down. Press gently to compact layers.
08 - Insert toothpicks at opposite corners to hold the sandwich together. Slice diagonally into halves or quarters for serving.
09 - Repeat the assembly process for the remaining 3 sandwiches. Serve immediately while bread remains crisp.