Preheat a skillet over medium heat.
Lightly butter one side of each slice of bread.
Toast the bread slices in the skillet, buttered side down, until golden brown and crisp, about 2-3 minutes per side. Remove and set aside.
Spread mayonnaise evenly on the unbuttered side of three toasted bread slices.
Place two slices of turkey breast on top of the mayonnaise on each of these three slices.
Add two slices of tomato on top of the turkey breast on two of the slices. Season tomatoes with salt and freshly ground black pepper.
Place two leaves of romaine lettuce on top of the tomato slices.
Add two slices of cooked turkey bacon on top of the lettuce on two of the slices.
Top each stack with the remaining toasted bread slices, mayonnaise side down, to form a double-decker sandwich.
Secure each sandwich with two toothpicks, one at each end.
Cut each sandwich diagonally into four triangular quarters.
Serve immediately.