Preheat oven to 350 degrees Fahrenheit and lightly coat a 9x13" pan with nonstick spray. For the crust, stir together graham crackers and melted butter. Spread evenly into the bottom of the pan. Bake for 4 minutes. Remove from oven and sprinkle 2 1/2 c. marshmallows over the top.
While the crust is baking, make the cookie layer by creaming together butter, brown sugar, white sugar, vanilla and eggs in the bowl of an electric mixer. Mix in flour, salt and baking soda. Stir in chopped chocolate bars. Grab a big handful of the cookie dough and flatten it in your hand. Place it on top of the marshmallows. Continue doing this until all the area is covered.
Bake for approximately 20 minutes, checking at about 15 minutes and every minute after that. Don't overbake! The bars will be done when the edges just barely start turning a golden brown. Remove from the oven and immediately press the chocolate bars, mini marshmallows and graham cracker pieces into the top. Let cool. I put mine in the fridge, just for about 30 minutes so the chocolate could solidify because it will melt when you press it into the hot cookie bars. But I like to store mine at room temperature, covered. Cut into squares and serve.