Roughly chop the graham crackers and Hershey's chocolate bar. Tear the mini marshmallows in half to prevent excessive spreading during baking.

In a medium bowl, whisk together the flour, salt, baking powder, baking soda, and cornstarch. Set aside.

In a large mixing bowl, beat the butter, granulated sugar, and brown sugar until light and creamy, about 2 minutes.

Add the egg and vanilla extract. Mix until fully combined.

Gradually add the dry ingredients to the butter mixture, mixing on low speed until just incorporated.

Reserve a handful of marshmallows for topping.

Fold the remaining marshmallows, chocolate chips, and chopped graham crackers into the dough.

Cover and refrigerate the dough for at least 1 hour.

Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.

Scoop approximately 2-tablespoon portions of dough and roll into balls. Place them 2 inches apart on the baking sheet.

Bake the Smores Cookies for 8 minutes.

Remove the tray and gently press reserved marshmallows and chocolate pieces onto the tops of the cookies.

Return to the oven and bake for an additional 2–3 minutes.

Allow the cookies to cool completely on the baking sheet before serving.