A grilled cheese sandwich has four variables and most of us only think about one of them. Here is the order that actually matters.
One, butter the bread not the pan
Butter on the pan burns before the cheese moves. Butter spread thin on the outer face of each slice browns evenly and gives you that glassy crust.
Two, grate the cheese
Slices melt from the outside in and leave a firm core. Grated cheese melts as one. Mix a sharp cheddar with something stretchy and you get both flavour and pull.
Three, low heat, patient hands
Medium low for six to seven minutes a side. High heat gives you a dark crust and cold cheese, which is the most common failure by a distance.
Four, rest it for a minute
Cut immediately and the filling runs out onto the board. Give it sixty seconds and it sets just enough to stay where it belongs.
Life is better between two slices. It is better still when the cheese is grated.