Butter one side of each bread slice generously, spreading all the way to the edges.
Heat a skillet or griddle over medium heat (not too high or the bread will burn before the cheese melts).
Place one slice of bread butter-side down in the skillet, add the cheese slices, then top with the second bread slice, butter-side up.
Cook for 3 minutes until golden brown, then carefully flip and cook another 2-3 minutes until the second side is golden and cheese is completely melted.
Remove from heat, let cool for 1 minute, slice diagonally, and serve immediately while the cheese is still gooey!