Unwrap the chocolate bars and divide each into 4 pieces to make a total of 16 pieces of chocolate; set aside.
Line a large baking sheet with foil for easy clean up. Arrange 16 of the graham cracker squares in a single layer on the cookie sheet.
Top each graham cracker with 2 marshmallows.
Toast the marshmallows for 1-3 minutes in the oven set to broil (525F degrees) until golden brown on top. Use the middle oven rack, not the top rack close to the broiler. Important: Do not walk away from the oven while the marshmallows are toasting. They can burn very fast! Keep a close eye on the marshmallows as they toast.
Remove the s’mores from the oven. Turn off the oven.
Place one square of chocolate on top of each s’more, then return the s’mores to the hot oven for 30 seconds to melt the chocolate.
Remove the s’mores from the oven and top each with the remaining 16 graham cracker squares.