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 and golden, turning occasionally. Drain on paper towels to remove excess grease.
03 - Lightly butter bread slices if desired. Toast in a toaster, toaster oven, or grill pan until golden brown and crispy.
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 first slice, place romaine lettuce leaves, followed by tomato slices, and half of the seasoned sliced chicken.
06 - Place a second slice of bread, mayonnaise side down, over the first layer. Add more lettuce, tomato slices, and 2 crispy bacon slices.
07 - Top with the third slice of bread, mayonnaise side down. Press gently to secure the layers.
08 - Insert toothpicks at opposite corners to hold the sandwich together. Slice diagonally into halves or quarters using a sharp knife.
09 - Repeat the assembly process for the remaining sandwiches. Serve immediately while bread remains crispy.