Heat a grill pan or a large heavy nonstick skillet over medium-low heat. Meanwhile, brush one side of each bread slice with some of the melted butter. Place half of the bread slices on a work surface, buttered side down; sprinkle with chocolate, covering the bread to within 1/4-inch of the crust. Top with remaining slices, buttered sides up. Place sandwiches, 2 at a time, in grill pan; weigh down top of sandwiches with a large heavy skillet.