Preheat the oven to 450 degrees.
Lay slices of the bread, butter side down, on an ungreased sheet pan. Sprinkle evenly with the chopped chocolate, covering the bread to within 1/4-inch of the crust. Cover each with a second slice of bread, butter side up. Press closed with the back of a spatula. Bake until the bread is golden and the chocolate begins to melt, about 5 to 7 minutes. Using a spatula, carefully turn each sandwich over and bake 5 to 7 minutes more. Let cool slightly before serving.