Into a big mixing bowl, add the room temperature butter, sugar, brown sugar, and vanilla extract, and cream it with an electric hand mixer for 2-3 minutes or until it's creamy and light in color.
Add the eggs and the egg yolk, and mix until combined and creamy.
Then, add the flour, cornstarch, baking soda, and kosher salt, and mix with the hand mixer until almost combined.
Add the chocolate chips, mini marshmallows, and graham cracker chunks, and fold with a spatula until the mixture is just combined.
Cover the bowl with plastic wrap and let the dough rest in the fridge for at least an hour. We let the dough rest in the fridge overnight.
Use a large cookie scooper to scoop out the cookies or scoop the cookies with a regular tablespoon, 3 tablespoons at a time.
Make a hole using your thumb in the middle of the cookie dough ball and stuff it with the marshmallow. We used ½ a marshmallow per cookie dough ball, but you can use a whole marshmallow.
Place the cookie dough balls onto a parchment paper-lined baking sheet and bake in a 180c (350f) preheated oven for 11-12 minutes. Immediately top them with more chocolate chips.
Let the cookies cool for 5 minutes, and then transfer them to a wire rack to cool completely.