In a medium bowl, mix together the Caesar dressing, mayonnaise, and garlic powder until well combined.
Add the shredded chicken and half of the Parmesan cheese to the bowl and toss until the chicken is evenly coated.
Butter one side of each bread slice and toast in a skillet over medium heat until golden brown, about 2-3 minutes per side.
To assemble, place romaine lettuce on four bread slices, top with the Caesar chicken mixture, sprinkle with remaining Parmesan, and top with the remaining bread slices.
Slice sandwiches in half and serve immediately while the bread is still warm and crispy.