Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until smooth and creamy.
Add the room temperature egg and vanilla extract, mixing until fully incorporated.
In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Fold in the semi-sweet chocolate chips.
Chill the dough for at least 30 minutes to allow the flour to hydrate properly.
Scoop the dough into rounded balls and place them on the prepared baking sheet.
Bake for 10-12 minutes until the edges are set.
Remove from the oven, press half a marshmallow into the center of each cookie, and sprinkle with crushed graham crackers and chopped chocolate bar.
Return to the oven for an additional 2-3 minutes until the marshmallow is soft and slightly toasted.