Preheat the oven to 350°Fahrenheit/175°Celsius and butter a 9 inch cast iron skillet.
In a large bowl, whisk together the cooled melted butter with the two sugars until smooth. Add the eggs, egg yolk and vanilla extract and whisk until smooth
In another large bowl, whisk together the flour with the baking powder, baking soda, salt and cornstarch.
Pour the dry ingredients into the wet ingredients and fold everything together using a rubber spatula, making sure there are no flour pockets. Pour the chocolate chips, marshmallows and graham crackers (though keep a small handful of each to the side!) until everything is evenly combined.
Pour the cookie dough into the greased pan and bake for 50-55 minutes. Take the skillet out 5 minutes before baking and sprinkle over the reserved marshmallows, chocolate chips and graham crackers. Place it back in the oven and bake for another 5 minutes. The sides should be a deep golden brown but the middle will still be gooey. If you want the cookie more cooked through then I recommend baking it for another 10 minutes, or until a toothpick inserted comes out with just a few crumbs.
Notes
My recipes have been developed and tested in cups and metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way.I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients before you start baking.Butter: I use European style butter, like Kerry Gold, in my recipes. Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Storage: Keep tightly wrapped in the fridge for up to 3 days.