01 - In a medium bowl, thoroughly combine grated cheddar, Gruyère, mozzarella, and fontina cheeses.
02 - Arrange bread slices on a flat surface. Using a spatula or butter knife, spread a thin layer of softened butter on one side of each slice.
03 - Drizzle ½ teaspoon truffle oil onto the unbuttered side of two bread slices.
04 - Evenly distribute the cheese mixture over the truffle-oiled sides of two bread slices.
05 - Sprinkle chopped chives and freshly ground black pepper over the cheese, if desired.
06 - Place the remaining bread slices on top of the filled sides, ensuring buttered side faces outward.
07 - Preheat a large skillet over medium-low heat. Add sandwiches and cook for 3 to 4 minutes per side, gently pressing until both sides are golden brown and cheese has melted.
08 - Transfer sandwiches from the skillet, let rest for 1 minute, then slice and serve promptly.