Preheat the oven to 350°F. Line a 9×9-inch baking pan with parchment paper.
Whisk together flour, oats, baking soda and salt in a medium bowl.
In a large bowl, beat together butter and brown sugar until light and creamy. Add vanilla extract and mix to combine. Add dry ingredients to the bowl and mix until a crumbly mixture forms.
Press 2/3 of the mixture into the prepared pan. Sprinkle chocolate, marshmallows and graham crackers over the crust and gently press down. Top with remaining cookie mixture.