While the grahams are in the oven, break up the chocolate bars into small pieces. When the grahams are removed from the oven, immediately cover with the chocolate pieces. It will quickly melt and you can smooth it evenly with the back of a wooden spoon. Place large marshmallows over grahams, two large marshmallows will fit on one graham cracker (1/4 of one whole graham cracker).
Place the marshmallow-covered grahams under the broiler for 30 to 60 seconds, watching carefully, until the marshmallows are toasted to your heart's desire.