Evenly apply butter to one side of each slice of bread.
Lay 2 slices of bread, butter side down, on a cutting board. Add a slice of white cheddar cheese on top of each.
Spoon half of the prepared macaroni and cheese onto each sandwich over the cheese slices. If desired, sprinkle with salt and pepper.
Place another slice of cheddar cheese over the macaroni. Cover with the remaining bread slices, butter-side up.
Heat a skillet over medium heat. Place the sandwiches in the skillet and cook for 3 to 5 minutes on each side, until the bread is golden brown and the cheese is melted.