1. Butter one side of each slice of bread generously.
2. In a skillet big enough to hold them, cook the slices butter side down over medium heat, just until pale gold on one side. Cover two of the slices with chocolate. Flip a naked slice on top, butter side up.
3. Cook, turning the sandwiches as necessary, until the chocolate is softened and the sandwiches are browned on both sides.
4. Serve immediately, sprinkled with a pinch of sea salt or a dusting of cinnamon sugar, if you like.