Preheat the oven to 350º. Line a 9x12" baking sheet with parchment paper and set aside.
In a medium sized bowl, mix together the all purpose flour, the baking soda, and the salt. Set aside.
In your mixer cream together the butter, sugars, vanilla, Ener-G egg (I don't pre-mix the egg, I just add the powder and water directly to the bowl), and additional Tablespoon of water. Mix until well combined and "fluffy."
Add the dry flour mix to the butter mixture, a little bit at a time. I mix it in 3 additions.
Add the graham cracker cereal, marshmallows, and chocolate chunks to the mixer. The batter will be very stiff at this point. If you have a powerful stand mixer like a Kitchen Aid you can give it a few stirs to incorporate. If not, stir in the ingredients by hand. It's ok if they're not fully incorporated—as the cookies bake they will all hold together.
Scoop the stiff batter onto the prepared baking sheet. Be sure that it is distributed evenly, although it will not "spread" with a spatula (it will spread while it bakes).
Sprinkle a few extra marshmallows across the top of the cookie bars.
Bake at 350º for 35 minutes. The cookies will be very soft when you remove them from the oven, but will continue cooking with residual heat while they cool. Give the baking sheet a good smack on the counter to "deflate" the cookie bars—this will help to give them a chewy texture.
Allow to cool fully before cutting. Cut into squares, serve, and enjoy! Store leftovers in an airtight container at room temperature for up to 3 days for best texture.