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. Toast in a toaster or grill pan until golden brown and crispy on both sides.
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, layer romaine lettuce leaves, 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, pressing gently to secure the layers.
08 - Insert toothpicks at opposite corners through all layers to hold the sandwich together for easier slicing and serving.
09 - Using a sharp knife, slice each sandwich diagonally into halves or quarters. Repeat assembly for remaining sandwiches. Serve immediately while bread remains crispy.