Roughly chop the graham crackers and Hershey’s bar, then tear the mini marshmallows in half so they bake more evenly.

Combine the flour, salt, baking powder, baking soda, and cornstarch in a bowl. Whisk well and set aside.

Using a stand mixer or hand mixer, beat the butter, granulated sugar, and brown sugar until creamy, about 2 minutes.

Mix in the egg and vanilla extract until combined, then scrape down the bowl and mix once more.

Gradually add the flour mixture to the butter mixture, mixing on low speed until just combined.

Reserve a small handful of torn marshmallows, then add the remaining marshmallows, chocolate chips, and graham crackers to the dough. Mix on low just until combined.

Refrigerate the dough for at least 1 hour so the cookies hold their shape while baking.

Preheat the oven to 350°F and line a baking sheet with parchment paper.

Scoop roughly 2 tablespoon portions of dough, roll into balls, and place them on the baking sheet about 2 inches apart.

Bake for 8 minutes.

Top each cookie with the reserved marshmallows and chopped Hershey’s bar pieces.

Return the cookies to the oven and bake for 2 to 3 more minutes until the edges are lightly golden.

If any marshmallows spread too much, use a round cookie cutter, spatula, or knife to nudge the edges back into shape while the cookies are still warm.

Leave the cookies on the baking sheet to cool completely so they can set properly.