Preheat oven to 350°F. Butter a cast iron skillet.
In a large bowl, combine butter, brown sugar, and white sugar. Cream together with a hand mixer. Add egg + egg yolk along with vanilla, mix until just combined.
In a separate bowl, mix flour, baking soda, and salt. Add to wet ingredients and mix by hand. Fold in ¾ths of the chocolate, graham cracker, and marshmallow. Doing this by hand prevents over-mixing.
Drop the cookie dough into the skillet and top with remaining chocolate, graham cracker, and marshmallow. Bake for 20-25 minutes, or until the cookie is lightly browned on the exterior but still a little soft in the center.
Let cool for 15 minutes and then serve with ice cream and chocolate syrup.
