Preheat a cast-iron skillet or griddle over medium-low heat. Spread 1 teaspoon butter onto one side of each bread slice.
Place 1 bread slice buttered side down in the skillet. Top with the cheese and the second bread slice buttered side up. Cook for 2 minutes. Flip carefully using a wide metal spatula, then cook for 2 minutes more on the second side; the bread should be barely colored. Repeat, cooking for about 2 minutes more on each side, until golden. Cut in half and serve immediately.