Over medium heat, warm 3/4 cup of mac & cheese In a small nonstick skillet. Once warm (about 3-4 minutes), shape mac & cheese into a square about the size of the bread and top with 2 slices of cheese. Turn off heat.
Butter 2 slices of bread and place both slices butter side down in another skillet over medium heat.
Top one of the bread slices with 2 slices cheese. When lightly browned turn off heat. Use spatula to transfer mac & cheese rectangle onto cheese covered bread in the skillet.
Cover with the other bread slice, toasted side up. Gently press together with spatula and transfer to a plate. Serve immediately.