01 - In a medium saucepan, bring water and salt to a boil. Add elbow macaroni and cook according to package directions until al dente. Drain and return the macaroni to the pot. Stir in the butter until melted. Add the whole milk, 1 1/2 cups of shredded cheddar cheese, black pepper, and optional dry mustard powder. Stir over low heat until the cheese is fully melted and the mixture is smooth and creamy. Remove from heat and allow to cool slightly.
02 - Lay out the 8 slices of bread. Butter one side of each slice. Place 4 of these slices, butter-side down, on a clean work surface. Distribute a generous portion of the prepared macaroni and cheese onto each of these 4 bread slices, followed by a sprinkle of the additional shredded cheddar cheese. Top each with one of the remaining bread slices, ensuring the buttered side is facing upwards.
03 - Heat a large nonstick skillet or griddle over medium heat. Carefully place the assembled sandwiches onto the hot skillet. Cook for 3–4 minutes per side, pressing gently with a spatula, until the bread is golden brown and crispy, and the cheese inside is thoroughly melted. Remove the sandwiches from the skillet, let them cool for a moment, then cut in half and serve warm.