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, about 5-7 minutes, turning occasionally. Drain on paper towels to remove excess grease.
03 - Lightly butter bread slices if desired and toast in a toaster or grill pan until golden brown and crisp.
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 romaine lettuce leaves, followed by tomato slices, and half of the seasoned sliced chicken.
06 - Top with a second slice of bread, mayonnaise side down. Add additional lettuce, tomato slices, and 2 slices of crispy bacon.
07 - Place the third slice of bread, mayonnaise side down, on top to finish the triple-decker structure.
08 - Insert toothpicks at opposite corners through all layers to hold the sandwich together for serving.
09 - Cut each sandwich diagonally into halves or quarters using a sharp knife. Serve immediately while bread remains crispy.