Whisk the flour, baking soda, cornstarch and salt together in a large bowl. Set aside. In a medium bowl, whisk the melted butter, brown sugar, and white sugar together until no brown sugar lumps remain. Whisk in the egg, then the egg yolk. Finally, whisk in the vanilla. Pour the wet ingredients into the dry ingredients and mix together with a large wooden spoon or silicone spatula. The dough will be very soft, yet thick. Fold in the chocolate chips. They may not stick to the dough because of the melted butter, but do your best to mix them in. Cover the dough and chill in the refrigerator for 2 hours or up to 3 days. Chilling is mandatory.

Take the dough out of the refrigerator and allow to slightly soften at room temperature for 10 minutes.

Preheat the oven to 325°F (163°C). Line two large baking sheets with parchment paper or silicone baking mats. Set aside.

Roll the dough into balls, about 3 Tablespoons of dough each. The dough will be crumbly, but the warmth of your hands will help the balls to stay intact. Place 8 balls of dough onto each baking sheet. Bake the cookies for 12 minutes. Remove the cookies from the oven and press some of the toppings into each cookie (the chocolate chunks, graham cracker pieces, and marshmallows). Place the cookies back into the oven for 2 more minutes. If desired for the last 30 seconds, turn the broiler on high to help toast the marshmallows. Or, after the 2 minutes, use a kitchen torch to toast the marshmallows once out of the oven.

Allow the cookies to cool on the cookie sheet for 10 minutes before moving to a wire rack to cool completely.