Pre-heat oven to 375℉. Spray 9" x 13" baking dish with cooking spray.
In a big bowl, mix together wet ingredients - butter, brown sugar, egg, vanilla extract, and milk.
Then add these dry ingredients into wet ingredients mixture - flour, baking soda and crushed graham crackers. Mix to combine.
Evenly spread mixture onto the bottom of the baking dish, creating a thin layer. Bake in the oven for 10 minutes.
Remove baking dish from oven, then sprinkle chocolate chips, marshmallow and small pieces of graham crackers onto the top.
Bring baking dish back into the oven and bake for 5-7 minutes, or until chocolate has melted. Optional: broil for an additional 1-2 minutes to brown the tops of marshmallows.
Remove from oven and let it cool completely.
Cut into 12 square pieces. Serve and enjoy!