01 - Evenly season the sliced cooked chicken breasts with salt and freshly ground black pepper on both sides.
02 - Arrange bacon slices in a cold skillet and cook over medium heat for 6-8 minutes, turning occasionally, until crispy and browned. Transfer to paper towels to drain excess grease.
03 - Lightly butter one side of each bread slice if desired. Toast in a toaster, toaster oven, or skillet until golden brown and crispy on both sides.
04 - For each sandwich, lay out 3 toasted bread slices. Spread mayonnaise (and Dijon mustard if using) generously on one side of each slice.
05 - On the first mayonnaise-coated slice, layer 2 romaine leaves, 2-3 tomato slices, and half of the seasoned chicken slices.
06 - Place a second bread slice, mayonnaise side down, over the first layer. Add 2 more lettuce leaves, additional tomato slices, and 2 crispy bacon strips.
07 - Top with the final bread slice, mayonnaise side down, pressing gently to secure all layers.
08 - Insert toothpicks through opposite corners of each sandwich to hold the stack together. Using a serrated knife, slice diagonally into halves or quarters for serving.
09 - Repeat the layering process for the remaining 3 sandwiches. Serve immediately while toast remains crispy.