Butter one side of each slice of bread generously. In a skillet, cook the slices of butter side down bread over medium heat, just until pale golden brown on one side.
Cover tow of the slices with chocolate. Flip a “raw” bread slice on top, butter side up.
Cook, turning the sandwiches as necessary, until the chocolate is softened and the sandwiches are browned on both sides.
Serve immediately, sprinkled with a pinch of sea salt or a dusting of cinnamon sugar.