Start by chopping the graham crackers into rough pieces along with the Hershey's chocolate bar. Tear the mini marshmallows in half—this prevents them from expanding excessively during baking, which helps maintain the cookie’s shape.

In a separate mixing bowl, whisk together the flour, cornstarch, salt, baking powder, and baking soda until well-combined. Set this dry mixture aside.

Using an electric mixer (either hand or stand), cream the room temperature butter, granulated sugar, and light brown sugar until the texture becomes smooth and creamy—about 2 minutes. Add in the egg and vanilla extract, and continue mixing until fully integrated. Scrape down the sides of the bowl to ensure everything is evenly incorporated, then give it another quick mix.

Slowly add the dry ingredients into the butter mixture, mixing on low speed until just combined. Be careful not to over-mix.

Set aside a small portion of the halved marshmallows for later use, then fold the remaining marshmallows, semisweet chocolate chips, and chopped graham crackers into the dough. Mix on low speed, ensuring everything is just blended.

Cover the dough and refrigerate for at least an hour, allowing the flavors to meld and the dough to firm up.

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper. Roll about two tablespoons of dough into balls and place them on the baking sheet, leaving approximately 2 inches of space between each ball to allow room for spreading.

Bake the cookies for 8 minutes. Remove the baking sheet and press a few of the reserved marshmallow pieces and chocolate bar chunks into the tops of the cookies. Return the cookies to the oven and bake for an additional 2-3 minutes, just until the marshmallows turn golden and the edges set.

If any cookies spread out too much due to excess marshmallow, use a round cookie cutter or knife to gently reshape them while still warm. Let the cookies cool completely on the baking sheet before transferring to a wire rack. Enjoy your smores cookies!