01 - Season the cooked chicken breast slices evenly with salt and black pepper on both sides.
02 - In a skillet over medium heat, cook the bacon slices until crispy and golden, about 5-7 minutes. Transfer to paper towels to drain excess grease.
03 - Lightly butter the bread slices and toast them 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, arrange romaine lettuce leaves, tomato slices, and half of the seasoned sliced chicken.
06 - Place a second slice of bread, mayonnaise side down. Top with additional lettuce, tomato slices, and 2 crispy bacon slices.
07 - Finish with the third slice of bread, mayonnaise side down, pressing gently to secure the 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 sandwiches. Serve immediately while bread is still crispy.