01 - Cook elbow macaroni in salted boiling water according to package directions. Drain and reserve.
02 - In a medium saucepan over medium heat, melt 2 tablespoons of butter. Incorporate the flour and whisk continuously for 1 minute to create a smooth roux.
03 - Gradually whisk in the whole milk, stirring constantly until the sauce begins to thicken, approximately 2 minutes. Add the shredded cheddar and mozzarella cheeses, stirring until fully melted and the sauce is smooth. Season with salt, pepper, and ground mustard, if desired.
04 - Add the cooked macaroni to the cheese sauce and stir until thoroughly combined. Remove from heat.
05 - Lay out the bread slices. Evenly spread a thin layer of softened butter on one side of each slice.
06 - Place 4 bread slices, buttered-side down, on a clean surface. Top each slice with one slice of cheddar or American cheese. Spoon a generous portion of the macaroni and cheese mixture over the cheese slices, spreading it evenly.
07 - Top each assembled sandwich with a remaining bread slice, buttered-side up.
08 - Heat a nonstick skillet or griddle over medium-low heat. Grill sandwiches for 3 to 4 minutes per side, pressing gently, until the bread is golden brown and the cheese is fully melted.
09 - Slice the grilled sandwiches and serve immediately.