Prepare macaroni according to package directions. Drain the macaroni in a colander and return to the pot.
Add butter, cheddar cheese, Parmesan cheese, American cheese, milk, salt, mustard powder, cayenne to the pot. Turn heat up to medium-low and stir until cheeses are melted and all ingredients are well combined.
Remove the pot from the heat and pour onto a parchment paper-lined sheet tray. Spread evenly with a spatula, cover with plastic wrap, and refrigerate for at least 30 minutes.
Once cooled, use a ring mold or rounded container roughly 4-inches (10 cm) in diameter to cut out 6-8 “buns.”
Place flour, eggs, and bread crumbs in three separate bowls.
Dip each macaroni bun in flour, then egg, and finally bread crumbs, coating evenly.
Preheat vegetable oil to 350°F (175°C).
Place the macaroni buns gently in oil and fry 3-4 minutes, until golden brown. Remove with a slotted spoon and drain on paper towel-lined plate.
Season hamburger patties liberally with salt and pepper. In a cast-iron skillet over high heat, sear patties to desired doneness, flipping halfway through.
Place burger between two mac and cheese buns and top with sliced onions and tomatoes.