Thinly slice the cheddar cheese so it melts consistently and provides an even layer of cheese throughout the sandwich.
Use day-old bread with a firm texture to help prevent the slices from becoming overly soft or breaking apart during cooking and cutting.
Gently melt the unsalted butter, keeping it ready for an even coating on the bread.
Brush the melted butter uniformly over one side of each bread slice.
Arrange the thin cheddar slices over one piece of bread, then place the second slice on top with its buttered surface facing outward.
Heat a skillet over medium-high heat. The pan is ready when a small drop of water immediately sizzles upon contact.
Set the assembled sandwich in the hot skillet with one buttered side against the cooking surface. Toast for 2–4 minutes, or until the underside becomes crisp and evenly golden brown.
Carefully turn the sandwich over and cook for an additional 2–4 minutes, allowing the second side to develop the same golden, crisp finish. Keep the sandwich intact while cooking so the melted cheese remains contained and the structure stays firm.
Transfer the finished grilled cheese to a cutting board and allow it to rest for 1–2 minutes. This brief cooling period lets the melted cheese settle slightly, helping the croutons hold together when sliced.
Using a sharp knife, cut the sandwich into approximately 1-inch squares.
Serve the grilled cheese croutons immediately while the bread remains crisp and the cheddar is still warm and creamy.