The following recipe is adapted from Kitchen Remix. It’s a bit messy on your work surface—be warned—but worth the cleanup.
1/4 cup mayonnaise
½ tbsp. packed light brown sugar
8 (½-inch-thick) slices sturdy white bread
About 2 tbsp. extra-virgin olive oil
4 oz. bittersweet chocolate baking disks (about ¾ cup)
In a small bowl, combine the mayonnaise and brown sugar. Arrange the bread slices on a work surface and spread the tops with the sweetened mayonnaise, to coat. Flip the slices over and brush the other sides with olive oil. Divide the chocolate among four bread slices, drizzle with a bit more olive oil, and sprinkle with a pinch of salt. Cover the four chocolate-topped bread slices with the remaining slices, oil-side down.
Heat a large cast-iron skillet over medium-low heat. Place two of the sandwiches in the hot skillet and cook until the bottoms are golden brown and caramelized, from three to four minutes. Flip the sandwiches and cook until the second sides are golden brown and the chocolate is melted, about three minutes.
Transfer the finished sandwiches to a plate, wipe the pan clean, and cook the remaining sandwiches the same way. Let the sandwiches cool for a couple of minutes before cutting them in half and serving hot.