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 one side of each bread slice if desired. Toast in a toaster or on a grill pan until golden brown and crispy.
04 - Lay out 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 (mayonnaise side up), layer romaine lettuce leaves, tomato slices, and half of the seasoned chicken slices.
06 - Place a second bread slice on top, mayonnaise side down. Add additional lettuce, tomato slices, and 2 crispy bacon slices.
07 - Top with the third bread slice, mayonnaise side down. Press gently to compact the layers.
08 - Insert toothpicks at opposite corners through all three layers to hold the sandwich together.
09 - Using a sharp knife, slice the sandwich diagonally into halves or quarters. Repeat assembly process for remaining 3 sandwiches. Serve immediately.