Preheat your oven to 350°F (175°C) to ensure even baking.
In a large mixing bowl, combine the softened unsalted butter, light brown sugar, and granulated sugar. Beat with an electric mixer on medium speed until light and fluffy, about 2-3 minutes.
Add the large egg and vanilla extract to the butter and sugar mixture. Mix until fully incorporated, scraping down the sides as needed.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and a pinch of salt. Gradually add this dry mixture to the wet ingredients, mixing just until combined. Avoid overmixing.
Gently fold in the chopped graham crackers, semi-sweet chocolate chips, and marshmallow bits using a spatula to evenly distribute them throughout the dough.
Portion the dough onto baking sheets lined with parchment paper using a cookie scoop or spoon, leaving space between cookies for spreading.
Bake the cookies in the preheated oven for 10-12 minutes, until edges are golden brown and centers remain soft.
Let the cookies cool on the baking sheets for about 5 minutes before transferring to wire racks to cool completely. Enjoy!