Lay out the 8 slices of crusty artisan bread on your work surface.

Spread butter on one side of each bread slice; the buttered side will be the outside of the sandwiches.

On the unbuttered side of four slices, evenly spread the 1 cup marinara (divide the sauce among the four slices).

Divide the 8 thin slices mozzarella (or about 1/3 cup shredded) and the 8 thin slices cheddar among the four sandwiches; on each sauced slice place 2 slices mozzarella and 2 slices cheddar.

If using basil, evenly divide and sprinkle the 4 Tablespoons chopped basil over the cheese on each sandwich.

Top each sandwich with the remaining bread slice, butter-side facing out, and press gently to flatten slightly.

Preheat a griddle or large skillet over medium heat. Place the sandwiches on the hot surface and cook until the bottom is golden brown and the cheese begins to melt, about 3–4 minutes; flip once and cook the second side until golden and the cheese is fully melted.

Remove the sandwiches from the griddle and serve immediately.