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. Transfer to paper towels to drain excess grease.
03 - Lightly butter one side of each bread slice. 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 slice, arrange lettuce leaves, tomato slices, and half of the seasoned sliced chicken.
06 - Place a second bread slice on top, mayonnaise side down. Add another layer of lettuce, tomato, and 2 slices of crispy bacon.
07 - Place the third bread slice on top, mayonnaise side down, to finish the triple-decker structure.
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 remains crispy.