From America's Test Kitchen Season 2: Simple Sandwiches
The perfect grilled cheese sandwich consists of evenly melted cheese between slices of lacy-crisp, buttery bread, but most don’t turn out that way. We set out to find the keys to getting that ideal sandwich every time.
Cutting slices from a block of cheese often produces uneven, scraggly pieces. We found that grating the cheese on a box grater enabled us to get an even layer of cheese onto the bread. Butter melted in the pan sometimes burned and didn’t always coat the bread evenly, so we opted to butter the bread rather than the pan. And to coat the bread evenly and prevent it from tearing, we melted the butter first. Finally, we learned that the secret of a crisp exterior is low heat; the longer it takes for the bread to become golden, the crispier the bread will become—these are grilled cheese sandwiches worth the wait.
Grilled cheese sandwiches are best served hot out of the pan, though in a pinch they can be held, unsliced, for about 20 minutes in a warm oven. If you want to make more than two sandwiches at once, get two skillets going or use an electric griddle, set at medium-low (about 250 degrees), for 10 minutes per side. There are endless additions to grilled cheese sandwiches, but the extras are best sandwiched between the cheese. Try a few very thin slices of baked ham or proscuitto, turkey breast, ripe, in-season tomato, or two or three tablespoons of caramelized onions. Condiments, such as Dijon mustard, pickle relish, or chutney, can be spread on the bread instead of sandwiched in the cheese.