Cook macaroni according to package directions. Drain and set aside.
In saucepan, melt butter over medium heat and stir in flour. Add milk slowly while stirring constantly. Stir in cheese and cook over low heat until cheese is melted and sauce is slightly thick. Pour over drained, cooked macaroni and stir to combine.
Pour macaroni and cheese into rectangular cake pan or baking sheet and smooth into even layer (it should be about 1/2-inch thick). Refrigerate pan for 1 hour.
Remove pan from refrigerator. Using upside down drinking glass or round cookie cutter, cut out 8 circles to create macaroni and cheese “buns.” If necessary, use knife to trace around circles.
In a skillet over medium heat, toast macaroni and cheese buns for 2 minutes on each side or until golden brown in color.
Divide ground beef into four 1/4-pound portions. Season with salt and pepper and grill 4 minutes on each side or until internal temperature reaches 145°F for medium or 155°F for medium well.
Place each hamburger patty between 2 macaroni and cheese buns and top with your choice of garnishes.