Heat a large grill pan or skillet over medium heat until water flicked onto the surface sizzles immediately. Butter one side of each slice of bread. Lay the slices butter-side up on the hot skillet and heat until the bottoms are lightly toasted. Flip, and immediately add one slice of cheese to each of the toasted sides of the slices. Let cook until the cheese begins to melt. Spoon crab salad onto two of the bread slices. (The amount you use will vary according to the size of the bread - you might have leftover salad.) Top each with the other two slices (cheese side towards the crab salad), and continue cooking, if necessary, to toast both sides of the sandwiches.