There are many paths to an ideal grilled cheese sandwich, but a cast iron skillet (plus a lid) will always be the least perilous. Nothing rivals cast iron for an evenly heated surface, which ensures an evenly browned sandwich, and covering the skillet speeds up the cheese-melting process, which avoids the most-common grilled cheese misfires: under-melted cheese or over-browned bread.
Grilled cheese aficionados fall into two camps: mayo or no mayo. We’re advocates for the former, spreading a thin layer of the emulsified fat on the outside of the sandwich before it’s cooked, which further encourages even browning and adds a subtle tangy flavor we love. If you’re not a mayo maniac, simply leave it out and add extra butter to the skillet as needed.