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, 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, place lettuce leaves followed by tomato slices. Arrange half of the seasoned chicken slices on top.
06 - Top with a second bread slice, mayonnaise side down. Layer additional lettuce, tomato slices, and 2 crispy bacon pieces.
07 - Place the third bread slice, mayonnaise side down, on top to finish the triple-decker construction.
08 - Insert toothpicks at opposite corners through all layers to hold the sandwich together.
09 - Using a sharp knife, slice the sandwich diagonally into halves or quarters. Serve immediately while bread remains crispy.
10 - Repeat the assembly steps for the remaining 3 sandwiches.