In a mixing bowl, combine the shredded or diced chicken breast with light mayonnaise, Greek yogurt, Dijon mustard, celery, red onion, and parsley.
Stir until well combined, and season with salt and pepper to taste.
Cover and refrigerate the chicken salad while you prepare the chaffles.
Step 2: Make the Chaffles
Preheat a waffle maker (preferably a mini waffle maker for sandwich-sized chaffles). Spray it lightly with cooking spray.
In a medium bowl, whisk together the eggs, shredded cheddar cheese, baking powder, and garlic powder (if using).
Pour a small amount of the egg and cheese mixture into the preheated waffle maker, spreading it evenly to fill the surface. Close the waffle maker and cook for 3-4 minutes, or until the chaffle is golden brown and crispy.
Remove the chaffle and repeat the process to make four chaffles in total.
Step 3: Assemble the Sandwich
Place one chaffle on a plate as the base. Add a generous scoop of the prepared chicken salad on top.
Layer with lettuce leaves and tomato slices, if desired.
Top with another chaffle to complete the sandwich.
Repeat the assembly for the second sandwich.
Step 4: Serve and Enjoy
Serve the sandwiches immediately while the chaffles are warm and crispy.