In a small bowl, whisk together mayonnaise, ketchup, relish, Dijon, garlic powder, and paprika to make the secret sauce. Chill in the fridge.

Bake or fry bacon slices until crispy. Drain on paper towels and set aside.

In a large bowl, combine the cold, leftover mac and cheese, eggs, shredded cheddar, and flour. Mix until it forms a cohesive, tacky mixture.

Preheat a waffle iron and grease well. Scoop 1/2 to 3/4 cup of the mac and cheese mixture into the iron. Cook for 5-8 minutes until deeply golden and crispy. Repeat to make 4 waffle buns.

Divide the ground beef into four 4-ounce portions, rolling them loosely into balls. Do not overwork the meat.

Heat a cast-iron skillet over high heat. Add a tiny bit of oil. Place beef balls in the pan, cover with parchment paper, and use a heavy spatula to smash them completely flat. Season with salt and pepper.

Cook for 90 seconds until a dark crust forms. Scrape under the patty to flip, add a slice of cheese, and cook for 1 more minute until melted.

Assemble the burger: Waffle bun, layer of secret sauce, smash patty with cheese, crispy bacon, second smash patty, more sauce, and top with the final waffle bun. Serve immediately.