Split graham cracker into 2 halves. On top of graham cracker squares place mini-marshmallows. Place under broiler for a few seconds until toasty. Remove and place 1/2 of a Hershey bar on top of marshmallow. The heat from the marshmallow will melt the chocolate bar. Place other 1/2 of graham cracker on top. Press lightly to flatten marshmallows. These may be made ahead and wrapped in plastic wrap until ready to heat and eat.