01 - Preheat oven to 375°F. Lightly coat a 12-cup muffin tin with nonstick cooking spray.
02 - Bring salted water to a boil and cook elbow macaroni until al dente. Drain and set aside.
03 - In a saucepan over medium heat, melt butter and stir in flour. Cook for 1 minute, stirring constantly. Gradually whisk in milk and cook until slightly thickened, about 2–3 minutes.
04 - Remove saucepan from heat and incorporate cheddar, Parmesan, Dijon mustard, salt, and pepper until smooth. Fold in drained macaroni and mix well. Set aside.
05 - In a skillet over medium-high heat, cook ground beef and onion together until beef is cooked through, roughly 5–6 minutes. Drain excess fat.
06 - Stir ketchup, yellow mustard, Worcestershire sauce, garlic powder, salt, and pepper into beef mixture. Cook for an additional minute, then remove from heat.
07 - Press each mini bun or flattened bread slice into prepared muffin tin, forming a cup that covers bottom and sides.
08 - Spoon about 1 tablespoon of cheeseburger mixture into each cup. Top with generous spoonful of macaroni and cheese.
09 - Sprinkle remaining shredded cheddar evenly over each cup.
10 - Bake for 15–18 minutes, until tops are golden and bubbly.
11 - Allow cups to cool slightly before removing from muffin tin. Serve warm.