Preheat the oven to 350 and grease an 13x9 baking dish.
In the bowl of an electric mixer, cream together butter and sugar on medium-high speed, until light in color and fluffed. 2-3 minutes.
Beat in egg and vanilla until mixed. Scrape sides and bottom of the bowl.
Set the speed to low, sift in the flour, baking soda, and salt. Mix the ingredients on low speed until just combined. Scrape sides and bottom of the bowl again and mix until all incorporated.
Spread cookie batter in the baking dish, evenly. (It will help if you grease your hands with some non-stick cooking spray so that the dough doesn't get stuck to your hands.)
Spread semi-sweet and white chocolate chips evenly over the cookie layer. Spread 1/3 of the bag of mini marshmallows.
Crumble: In a medium bowl, combine brown sugar, white sugar, and flour. Mix well until all ingredients are incorporated evenly.
Pour in melted butter and carefully fold it in with a large fork. You will want to be careful so you don’t over-mix. Crumb topping should look like large crumbs and not paste.
Spread crumble all over chocolate and marshmallows, evenly and carefully not to mush it.
Bake for 30-33 minutes. Take it out of the oven.
Turn the broiler on to lower setting.
Spread remaining marshmallows over the top, covering completely and evenly. Place it in the oven to toast the marshmallows. Keep an eye on it, don't walk away, because they will toast quickly.
Let it cool completely before cutting.
Cut the cookie bars with a greased knife and then, drizzle hot fudge over it and sprinkle graham cracker cookie crumbles on top.