1/2 stick (1/4 cup) unsalted butter, softened
16 (1/2-inch-thick) baguette slices, cut on a long diagonal
1 (3- to 4-ounce) fine-quality bittersweet chocolate bar (no more than 70% cacao if marked), broken into 1/2-inch pieces
Generously butter 1 side of each baguette slice. Place 8 slices, buttered sides down, on a work surface and cover each slice with chocolate, leaving a 1/4-inch border around edge. Top with remaining 8 slices, buttered sides up, to make 8 sandwiches.
Heat a dry 12-inch heavy skillet (not nonstick) over moderate heat until hot but not smoking, then fry 4 sandwiches, turning over once, until golden, 3 to 4 minutes total. Transfer sandwiches to a plate and wipe skillet clean. Fry remaining sandwiches in same manner. Serve warm.