Soak romaine hearts in ice water for 5 minutes.
While it soaks, add remaining caesar dressing ingredients into a shallow non-glass bowl or blender. Use an immersion blender or blend on medium to high speed to emulsify ingredients. Once smooth and combined, add in ½ of measured parmesan and stir. Chill in fridge
Smash chicken with a rolling pin of meat tenderizer to thin out to max 1 inch thick. Mix together blackened seasoning and rub onto the front and back of chicken.
Melt butter in skillet and sear chicken on medium-high for 5 minutes on both sides.
Put chicken aside to rest and toss together lettuce and dressing, top with sliced chicken, remaining parmesan, and croutons. Enjoy!