Heat the milk in a pan over medium heat. (Or in the microwave, one minute at a time).
Mix the cornstarch with just enough water to dissolve it.
When the milk is almost boiling, turn down to maintain the heat but prevent it from boiling, add the cornstarch mixture and stir in well. Continue stirring until it is well combined.
When the sauce is evenly thickened, add the butter. Salt and pepper to taste.
Butter each slice of sandwich bread on one side.
Heat a pan over low-medium heat. Place the first piece of bread, butter side down, place one slice of cheese on top, then a scoop of macaroni and cheese, and finally the second piece of bread with the butter out. Grill the sandwich on both sides until golden brown. Repeat with remaining bread. (You might have leftover macaroni and cheese, luckily it's a delicious treat on its own!)