01 - Season the cooked chicken breast slices evenly with salt and black pepper.
02 - In a skillet over medium heat, cook the bacon until crispy. Drain on paper towels to remove excess grease.
03 - Lightly butter the bread slices and toast them until golden brown and crispy, if desired.
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, layer romaine lettuce, tomato slices, and half of the sliced chicken.
06 - Top with a second slice of bread, mayonnaise side down. Add more lettuce, tomato slices, and 2 slices of crispy bacon.
07 - Place the third slice of bread, mayonnaise side down, on top to close the triple-decker sandwich.
08 - Secure the sandwich with toothpicks at opposite corners to prevent layers from shifting.
09 - Slice the sandwich diagonally into halves or quarters for easy serving.
10 - Repeat the assembly process for the remaining sandwiches. Serve immediately while bread remains crispy.