Preheat oven to 350° F. Grease a 12-inch cast iron skillet and set aside.
In a large bowl, combine melted butter, brown sugar, and granulated sugar. Stir until smooth. Add the eggs and vanilla and mix until combined.
Add the flour, graham cracker crumbs, baking soda, and salt. Stir until flour disappears. Stir in the chocolate chips and 1/2 cup of the mini marshmallows.
Spread batter evenly in prepared cast iron skillet. Sprinkle the top with a little sea salt.
Bake for 20 minutes or until golden brown. Remove from the oven and top with the mini marshmallows. Return to the oven and bake for 4 to 5 more minutes, until marshmallows are puffy and soft. Turn the broiler on and broil just until the marshmallows are toasted. Don’t walk away, watch them closely. They will brown quickly.
Remove from the oven and gently place the graham cracker pieces and Hershey’s squares on top of the cookie. Let cool for 10 to 15 minutes on a cooling rack. Cut into triangles and serve!