When I first heard about this kind of sandwich, I expected it to be gross. It was anything but! We added the bacon to the recipe and my grown boys love this sandwich. I mean, Bacon with grilled cheese.... Doesn't take long to make once the mac and cheese is prepared. Usually have all of this on hand too. The bread is nice and crusty with cheese on the outside but the macaroni inside makes this feel like a real meal. Forget your diet, and enjoy.
Make mac and cheese with milk and butter according to directions on the box. If using cold leftovers, heat a tad in microwave, then combine with the crumbled cooked bacon.
Heat a non-stick griddle or sauté pan on med-hi heat.
Butter one side of one bread slice. Sprinkle shredded cheese onto the buttered side of the bread with intention that most of the cheese will stick to the butter. Lay the slice, cheese side down on the hot pan. Some cheese may fall away. It might be a tad messy. That's ok.
Load a pile of the mac & cheese mixture onto the top of the bread slice in the pan. Spread out evenly on the bread. Top with a second slice of bread. Press down a little with a spatula on the top bread.
Spread butter on the top of the top slice of bread, and sprinkle more shredded cheese on it, like you did the first slice.
Toast on med heat the first side of the sandwich until golden. Don't burn the cheese. Flip over the sandwich with spatula onto the second side. Again some of your cheese on the second side may fall a bit or be messy. This is ok. Brown the second side. Ready to serve.