I used to always use butter on the outside of my grilled cheese sandwiches, until i tried mayo on the outside. Now i'll never go back to butter, the mayo gives it a wonderful sheen and crunch without being too cooked
mmm... never tried mayo before... i like extra cheese on my grilled cheese.. the trick to getting it all melty is to use a lid while cooking, makes the perfect melted cheese and still crispy
I personally think butter gives better flavor and browning. For cheese I like gruyere, machego, havarti, american, and cheddar. As for other ingredients I love adding some sort of thinley sliced meat and a fruit with some acidity or sweetness, The meat can be ham, prochuto, guanciale, or something else. For the fruit i like tomato, apple, pear, or something else. To get a crispy outside and melty inside just use a low heat.