01 - Cook elbow macaroni in boiling salted water until al dente, following package directions. Drain thoroughly.
02 - Melt 1 tablespoon of butter in a medium saucepan over medium heat. Add milk and bring to a gentle simmer.
03 - Stir the cooked macaroni, shredded cheddar, shredded mozzarella, salt, and pepper into the simmering milk. Continue stirring until the cheeses have melted and the sauce is creamy. Remove from heat.
04 - Lay out 8 slices of bread. Evenly spread softened butter on one side of each slice.
05 - Place 4 bread slices, buttered side down, onto a clean work surface. Top each with a slice of sharp cheddar cheese.
06 - Spoon approximately 1/4 cup of the prepared macaroni and cheese mixture over the cheddar cheese on each sandwich base.
07 - Top each filled slice with the remaining bread slices, ensuring the buttered side faces upwards.
08 - Heat a large skillet or griddle over medium-low heat. Cook each sandwich for 3 to 4 minutes per side, applying gentle pressure with a spatula, until the bread is golden brown and the cheese is fully melted.
09 - Slice the grilled sandwiches and serve immediately.