I was skeptical at first but can now tell you from making a batch of these yesterday that they are PERFECT for an open house. I AM somewhat of a purist when it comes to my S'mores with all my Girl Scout and Youth Group camping experience. Yes, they are infinitely better around a real camp fire. But these are just as tasty and can be prepared a whole day in advance.
Preheat over to 400 degrees.
line cookie sheet with aluminum foild. Place about six or eight graham crackers on the foil. Top each one with a regular size marshmallow. Heat in the oven for 5 minutes or until the marshmallow is lightly brown on top.
Add a half of a bar of chocolate and top with the second graham cracker. Mash lightly to set it all together.. If you're short on chocolate a quarter of a bar will work. Put back into the oven to heat the chocolate a bit. Watch carefully because you don't want a melted mess on your hands. Just a little softening of the chocolate is best.
For some reason I can't explain it takes a really long time for the chocolate to set up once you take it out of the oven. This is one time it actually is better a few hours later than eating it immediately. Plan ahead. If you make them a day ahead you can even cut them into fourths for bite-sized pieces.