I use a 9"x9" pan that has a removable collar similar to a cheesecake pan. I also put a piece of parchment paper lining the collar. I have no idea why on earth I didn't with this batch. Let's blame it on the Shiny Squirrel and move on.
Put about half of your brownie batter in the bottom of the pan. Layout your graham crackers leaving a bit a space between the crackers and around the edge. This seems to help hold the brownies together in the end.
Top with a layer of milk chocolate bars. Growing up we always made our S'mores with plain Hersey bars. If you used some other brand of chocolate go for it!
Since taking these pictures I have found using a small ice cream scoop to add the top layer of batter works well. I put a scoop of batter on top of each 'mallow and let is side down the sides.
Usually it will take a bit longer to get these brownies baked with the additions. Just keep an eye on them to not burn.
Cool the brownies completely before slicing. They are best served at room temperature so that the chocolate is melty and the marshmallows are gooey.
Don't use a napkin - lick your fingers!