Preheat the oven to 350 degrees and line a 9x9 square with parchment paper or non-stick foil, or coat with baking spray. Set aside. 

Mix graham cracker crumbs and melted butter together. Once combined, press the graham crackers into the bottom of the prepared baking pan. Set aside. 

In the bowl of a standing mixer (or with a hand mixer), beat the butter until light and fluffy, about 1 minute. 

Add in both the brown sugar and granulated sugar, and cream the mixture until light and fluffy, about 1 minute. 

Next, add in the egg. Mix until combined. 

Add in baking soda, salt, and vanilla extract, mixing until combined.

Add in graham cracker crumbs and flour, and mix until just combined. 

Lastly, add in the milk chocolate chips and the semi-sweet chocolate chips and mix until combined. 

Divide the cookie dough in half. Press half of the dough in an even layer on top of the graham cracker crust. 

Using two spoons, dollop the marshmallow fluff on top of the cookie dough. Then, spread the marshmallow fluff as best as you can into an even layer on top of the cookie dough. (Dipping the spoons in hot/warm water will help prevent the fluff from sticking to the spoons as much. I keep a bowl of hot/warm water next to me and dip the spoons as needed during the marshmallow step.)

Lastly, spread the remaining cookie dough on top of the marshmallow fluff. (Take small pieces of cookie dough, flatten them in your hand, and then press the dough on top of the fluff. I’ve found this to be much easier then trying to spread the dough all at once.) It’s just fine if some of the marshmallow fluff is not fully covered. 

Bake for about 25-30 minutes, or until the bars are lightly golden brown and the center is set. If the edges are getting dark too quickly, tent a piece of foil over the top of the bars and continue cooking (I usually do this at the 20 minute mark or so). 

Sprinkle with sea salt, if desired.

Allow to cool before cutting and serving. 

Store in an airtight container at room temperature or in the fridge.