In the bowl of a stand mixer, mix butter and both sugars on medium speed until fluffy. Add the eggs and vanilla, and mix well. Next, add the salt, baking soda, flour, and graham cracker crumbs. Mix on low speed at first, until ingredients are mostly combined, and then mix on medium speed until completely incorporated. Stir in chocolate chunks and marshmallow bits, if using. Spread cookie dough on top of the graham cracker crust in the cast iron skillet. Sprinkle with more chocolate chunks. Bake for 25-30 minutes, or until slightly golden-brown on top.