01 - Preheat oven to 375°F (190°C). Lightly coat a 12-cup muffin tin with nonstick spray.
02 - Bring a pot of salted water to a boil. Cook macaroni until al dente according to package directions. Drain and set aside.
03 - In a saucepan over medium heat, melt butter. Stir in flour; cook and stir for 1 minute. Gradually whisk in milk. Cook, stirring, until thickened slightly, about 2–3 minutes.
04 - Remove saucepan from heat. Stir in cheddar, Parmesan, Dijon mustard, salt, and pepper until smooth. Fold in cooked macaroni and combine thoroughly. Set aside.
05 - In a skillet over medium-high heat, cook ground beef and onion until beef is browned and fully cooked, about 5–6 minutes. Drain excess fat.
06 - Stir ketchup, yellow mustard, Worcestershire sauce, garlic powder, salt, and pepper into cooked beef mixture. Cook for 1 additional minute. Remove from heat.
07 - Press each bun or bread slice into the muffin tin cups to create bases. Ensure bread covers bottom and sides of each cup if using sandwich bread.
08 - Spoon approximately 1 tablespoon cheeseburger mixture into each bread cup. Top with generous spoonful of mac and cheese.
09 - Sprinkle remaining shredded cheddar evenly over each prepared cup.
10 - Bake in preheated oven for 15–18 minutes, until tops are golden and cheese is bubbly.
11 - Allow cups to cool slightly. Remove from muffin tin and serve warm.