Place a sheet of parchment paper or waxed paper over a cookie sheet. Chop the chocolate and place in a double boiler or a glass bowl set over a pot of water. Bring the water to a low boil and stir the chocolates until they are completely melted. Pour the melted chocolate over the parchment paper in a rough rectangle. Sprinkle the graham cracker pieces over the chocolate making sure to give each piece a little press. Do the same with the marshmallows and then leave the bark to set for at least two hours.