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. The mixture will be thin. Pour into the dry ingredients and mix together with a large wooden spoon or silicone spatula. The dough will be very soft, thick, and appear greasy. Fold in the chocolate chips. The chocolate chips may not stick to the dough because of the melted butter, but do your best to combine them.

Cover the dough and chill in the refrigerator for at least 2-3 hours or up to 3 days. I highly recommend chilling the cookie dough overnight for less spreading.

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

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

Using a cookie scoop or Tablespoon measuring spoon, measure 3 scant Tablespoons (about 2 ounces, or 60g) of dough for XL cookies or 2 heaping Tablespoons (about 1.75 ounces, or 50g) of dough for medium/large cookies. Roll into a ball, making sure the shape is taller rather than wide—almost like a cylinder. This helps the cookies bake up thicker. Repeat with remaining dough. Place 8–9 balls of dough onto each cookie 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 2-3 more minutes or until toasted on top. 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.

Cool on the baking sheet for 10 minutes. After 10 minutes of cooling on the baking sheets, transfer cookies to a wire rack to cool completely.

Cookies stay fresh covered at room temperature for up to 1 week.