01 - Cook elbow macaroni according to package instructions, drain thoroughly, and set aside.
02 - In a saucepan over medium heat, melt unsalted butter. Stir in all-purpose flour and cook, whisking constantly, for 1 minute until slightly golden. Gradually whisk in whole milk, cooking until sauce thickens.
03 - Add shredded sharp cheddar cheese and ground mustard (if using) to the milk mixture. Stir conistently until the sauce is smooth. Fold in the cooked macaroni, seasoning with salt and black pepper, then turn off the heat and allow to cool slightly.
04 - Lay out half the bread slices. Spread a generous layer of the prepared mac and cheese onto each. Place remaining bread slices over the mac and cheese to form sandwiches.
05 - In a shallow bowl, whisk together large eggs, whole milk, garlic powder, onion powder, salt, and black pepper until smooth and evenly combined.
06 - Carefully dip each assembled sandwich into the egg custard, ensuring both sides are coated without becoming saturated or soggy.
07 - Heat unsalted butter in a large nonstick skillet or griddle over medium heat. Fry the custard-coated sandwiches in batches, cooking for 3 to 4 minutes per side until both sides are golden brown and interiors are heated through.
08 - Slice sandwiches if desired. Serve while hot, garnished with chopped chives, extra grated cheese, or hot sauce.